curl_dofree: allow free(NULL)

curl_dofree: allow free(NULL)

Previously this memdebug free() replacement didn't properly work with a
NULL argument which has made us write code that avoids calling
free(NULL) - which causes some extra nuisance and unnecessary code.
Starting now, we should allow free(NULL) even when built with the
memdebug system enabled.

free(NULL) is permitted by POSIX