[Bug d/123349] gdc: buffer overflow detected with -defaultlib=

[pinskia at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123349

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
OPT_debuglib_ has the same bug.

Also:
          if (defaultlib != NULL)
            free (CONST_CAST (char *, defaultlib));

Can just be `free (CONST_CAST (char *, defaultlib));` without the null check.
free is a no-op for null.