tests: platform: Fix TEST(GetStateSensorReadings, testBadDecodeResponse)

Fix up an off-by-one error in the test suite. I modified the test to
instead report more sensors than were present in the message buffer and
received the following from ASAN:

```
=================================================================
==297936==ERROR: AddressSanitizer: stack-buffer-overflow on address 0xffffc18300b9 at pc 0xffffa04cae2c bp 0xffffc182fe40 sp 0xffffc182fe58
READ of size 1 at 0xffffc18300b9 thread T0
    #0 0xffffa04cae28 in pldm_msgbuf_extract_uint8 ../src/msgbuf/../msgbuf.h:129
    #1 0xffffa04cae28 in decode_get_state_sensor_readings_resp ../src/platform.c:804
    #2 0x2c0278 in GetStateSensorReadings_testBadDecodeResponse_Test::TestBody() ../tests/libpldm_platform_test.cpp:843
    #3 0xffffa03ba0f8 in void testing::internal::HandleExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*) (/lib64/libgtest.so.1.12.1+0x4a0f8)
    #4 0xffffa03a6680 in testing::Test::Run() (/lib64/libgtest.so.1.12.1+0x36680)
    #5 0xffffa03a688c in testing::TestInfo::Run() (/lib64/libgtest.so.1.12.1+0x3688c)
    #6 0xffffa03a6c30 in testing::TestSuite::Run() (/lib64/libgtest.so.1.12.1+0x36c30)
    #7 0xffffa03b2dd0 in testing::internal::UnitTestImpl::RunAllTests() (/lib64/libgtest.so.1.12.1+0x42dd0)
    #8 0xffffa03b1998 in testing::UnitTest::Run() (/lib64/libgtest.so.1.12.1+0x41998)
    #9 0xffffa0400940 in main (/lib64/libgtest_main.so.1.12.1+0x940)
    #10 0xffff9f80b584 in __libc_start_call_main (/lib64/libc.so.6+0x2b584)
    #11 0xffff9f80b65c in __libc_start_main@@GLIBC_2.34 (/lib64/libc.so.6+0x2b65c)
    #12 0x2b2a6c in _start (/mnt/host/andrew/src/openbmc/libpldm/build/tests/libpldm_platform_test+0x2b2a6c)

Address 0xffffc18300b9 is located in stack of thread T0 at offset 297 in frame
    #0 0x2bfe90 in GetStateSensorReadings_testBadDecodeResponse_Test::TestBody() ../tests/libpldm_platform_test.cpp:811

  This frame has 14 object(s):
    [48, 49) 'retcompletion_code' (line 831)
    [64, 65) 'retcomp_sensorCnt' (line 832)
    [80, 84) 'rc' (line 819)
    [96, 100) '<unknown>'
    [112, 116) 'stateField' (line 827)
    [128, 132) 'retstateField' (line 833)
    [144, 148) '<unknown>'
    [160, 168) '<unknown>'
    [192, 200) '<unknown>'
    [224, 232) '<unknown>'
    [256, 264) '<unknown>'
    [288, 297) 'responseMsg' (line 815) <== Memory access at offset 297 overflows this variable
    [320, 336) 'gtest_ar' (line 822)
    [352, 368) 'gtest_ar' (line 847)
HINT: this may be a false positive if your program uses some custom stack unwind mechanism, swapcontext or vfork
      (longjmp and C++ exceptions *are* supported)
SUMMARY: AddressSanitizer: stack-buffer-overflow ../src/msgbuf/../msgbuf.h:129 in pldm_msgbuf_extract_uint8
Shadow bytes around the buggy address:
  0x200ff8305fc0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x200ff8305fd0: 00 00 00 00 00 00 00 00 00 00 00 00 f1 f1 f1 f1
  0x200ff8305fe0: 00 00 f3 f3 00 00 00 00 00 00 00 00 00 00 00 00
  0x200ff8305ff0: 00 00 f1 f1 f1 f1 f1 f1 01 f2 01 f2 04 f2 f8 f2
  0x200ff8306000: 04 f2 04 f2 04 f2 00 f2 f2 f2 00 f2 f2 f2 00 f2
=>0x200ff8306010: f2 f2 00 f2 f2 f2 00[01]f2 f2 f8 f8 f2 f2 00 00
  0x200ff8306020: f3 f3 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x200ff8306030: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x200ff8306040: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x200ff8306050: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x200ff8306060: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
==297936==ABORTING
```

Signed-off-by: Andrew Jeffery <[email protected]>
Change-Id: Ib63254bd345d28924aee47eb230bcbea0c88811a

tests/libpldm_platform_test.cpp

@@ -842,7 +842,7 @@ TEST(GetStateSensorReadings, testBadDecodeResponse)
            (sizeof(get_sensor_state_field) * comp_sensorCnt));
 
     rc = decode_get_state_sensor_readings_resp(
-        response, responseMsg.size() - hdrSize + 1, &retcompletion_code,
+        response, responseMsg.size() - hdrSize, &retcompletion_code,
         &retcomp_sensorCnt, retstateField.data());
 
     EXPECT_EQ(rc, PLDM_ERROR_INVALID_LENGTH);