6502tube: fix behavior of opcode #2.

This was a Klaus Dormann test suite failure.  See
https://www.stardot.org.uk/forums/viewtopic.php?p=440921#p440921 and
#239

Opcode 02 is HLT (undocumented) on the NMOS 6502 but is a NOP on the
65C02.  The Dormann test suite tested that this opcode causes the byte
immediately after it to be skipped rather than executed, i.e. it is a
two-byte instruction.  This commit makes that so and fixes the test
failure.

src/6502tube.c

@@ -593,8 +593,8 @@ void tube_6502_exec()
                 case 0x02:
                         if (dbg_tube6502)
                             debug_trap(&tube6502_cpu_debug, oldtpc, 1);
-                        else
-                            polltime(1);
+                        polltime(2);
+                        readmem(pc++);
                         break;
 
                 case 0x04:      /*TSB zp */