libiberty: Always return NULL if d_add_substitution fails.

libiberty: Always return NULL if d_add_substitution fails.

d_add_substitution can fail for various reasons, like when the subs array
is full. If d_add_substitution fails d_substitution should return NULL
early and not try to continue. Every other call of d_add_substitution
is handled in the same way.

libiberty/ChangeLog:

	* cp-demangle.c (d_substitution): Return NULL if d_add_substitution
	fails.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@247055 138bc75d-0d04-0410-961f-82ee72b054a4