Check for valid PC when stepping in the ARM sim debugger.

Check for valid PC when stepping in the ARM sim debugger.


BUG=v8:2134
TEST=

Committed: https://code.google.com/p/v8/source/detail?r=11821

[Yang, 2012-06-14 15:02:28]

Avoid crashing with a segfault when stepping further when we already returned
from the JIT code executed by the simulator.

When we return from code executed by the simulator, the PC is set to a sentinel
value (-2). If we try to decode the instruction at this point, dereferencing the
PC leads to a segfault.

This issue does not exist on MIPS.

PTAL.

[Jakob Kummerow, 2012-06-14 15:04:15]

lgtm