libiberty: Make `objalloc_free' `free'-like WRT null pointer

libiberty: Make `objalloc_free' `free'-like WRT null pointer

Inspired by a suggestion from Jan Beulich to make one of `objalloc_free'
callers `free'-like with respect to null pointer argument handling make
the function return with no action taken rather than crashing when such
a pointer is passed.  This is to make the API consistent with ISO C and
to relieve all the callers from having to check for a null pointer.

	libiberty/
	* objalloc.c (objalloc_free): Don't use the pointer passed if
	null.