(default_compilers): Add missing comma between strings, so that traditional compilers can compile gcc.c.

(default_compilers): Add missing comma between strings, so
that traditional compilers can compile gcc.c.
(main): Don't pass strcpy or strlen a NULL pointer if some of the
spec strings in default_compilers are NULL.
(validate_all_switches): Don't dereference a NULL pointer if some of
the spec strings in defalut_compilers are NULL.

From-SVN: r2039