parrot --gc-threshold coredumps when given no threshold

[dukeleto]

If no GC threshold is given, we get a coredump:

Core was generated by `./parrot --gc-threshold'.
Program terminated with signal 11, Segmentation fault.
#0  is_all_digits (argc=2, argv=0xbf97d694) at src/main.c:184
184	    for (; *s; ++s)
(gdb) bt
#0  is_all_digits (argc=2, argv=0xbf97d694) at src/main.c:184
#1  parseflags_minimal (argc=2, argv=0xbf97d694) at src/main.c:408
#2  main (argc=2, argv=0xbf97d694) at src/main.c:141
(gdb) 

This seems to be because is_all_digits doesn't like being passed a null string.

[chromatic]

Fixed in r49246. See also TT #1150.