Exception is thrown if $pc is not a readable address

I occasionally get this crash when running funfuzz:

Traceback (most recent call last):
  File "/usr/local/fbcode/gcc-5-glibc-2.23/lib/python3.6/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/local/fbcode/gcc-5-glibc-2.23/lib/python3.6/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/home/gulshan/virtualenvs/funfuzz2/lib/python3.6/site-packages/funfuzz/js/loop.py", line 298, in <module>
    sys.argv[1:], create_collector.make_collector(), False)
  File "/home/gulshan/virtualenvs/funfuzz2/lib/python3.6/site-packages/funfuzz/js/loop.py", line 179, in many_timed_runs
    js_interesting_options.jsengineWithArgs, logPrefix, False, env=env)
  File "/home/gulshan/virtualenvs/funfuzz2/lib/python3.6/site-packages/funfuzz/js/js_interesting.py", line 173, in __init__
    crashInfo = CrashInfo.CrashInfo.fromRawCrashData(out, err, pc, auxCrashData=auxCrashData)
  File "/home/gulshan/virtualenvs/funfuzz2/lib/python3.6/site-packages/FTB/Signatures/CrashInfo.py", line 219, in fromRawCrashData
    result = GDBCrashInfo(stdout, stderr, configuration, auxCrashData)
  File "/home/gulshan/virtualenvs/funfuzz2/lib/python3.6/site-packages/FTB/Signatures/CrashInfo.py", line 954, in __init__
    crashAddress = GDBCrashInfo.calculateCrashAddress(self.crashInstruction, self.registers)
  File "/home/gulshan/virtualenvs/funfuzz2/lib/python3.6/site-packages/FTB/Signatures/CrashInfo.py", line 1021, in calculateCrashAddress
    raise RuntimeError("Invalid instruction: %s" % instruction)
RuntimeError: Invalid instruction: /home/gulshan/virtualenvs/funfuzz2/lib/python3.6/site-packages/funfuzz/util/gdb_cmds.txt:28:

The reason this happens is that $pc is not a readable address. In the gdb_cmds.txt script, we run the command x/8i $pc. When I load the coredump manually in GDB and run this command, I get:

> x $pc
0x7f7466df4d56: Cannot access memory at address 0x7f7466df4d56

Then calculateCrashAddress function throws an exception because the self.crashInstruction is not in the valid format (it doesn't match the regex).

Start with funfuzz/util/gdb_cmds.txt and FTB/Signatures/CrashInfo.py, especially GDBCrashInfo.init and calculateCrashAddress. Reproduce the unreadable-$pc GDB output shown in the issue and trace how it becomes self.crashInstruction. Done means this output no longer causes an unhandled RuntimeError during crash processing, with the relevant behavior covered by the existing tests if available.