[PAL] Fix attestation LibOS regression test for UBSan

UBSan detects an uninitialized boolean variable on a DCAP SGX machine on
the remote attestation example, failing with the message:

    error: ubsan: load of invalid value for bool or enum: 100
    error: ubsan: ../pal/src/host/linux-sgx/pal_misc.c:724:43

This happens because `bool linkable` variable is assigned only in the
EPID attestation scheme, but left unassigned in the DCAP scheme. This
commit fixes this (though it's not a bug since DCAP never uses
`linkable` anyway). Hence, UBSan doesn't complain anymore.

Signed-off-by: Dmitrii Kuvaiskii <[email protected]>

pal/src/host/linux-sgx/pal_misc.c

@@ -702,7 +702,7 @@ int _PalAttestationQuote(const void* user_report_data, size_t user_report_data_s
 
     enum sgx_attestation_type attestation_type;
     sgx_spid_t spid;
-    bool linkable;
+    bool linkable = false;
 
     ret = parse_attestation_type(g_pal_public_state.manifest_root, &attestation_type);
     if (ret < 0) {