ld: fix -std=gnu23 compatibility wrt _Bool

ld: fix -std=gnu23 compatibility wrt _Bool

GCC trunk now defaults to -std=gnu23. We return false in a few places
which can't work when true/false are a proper type (_Bool). Return NULL
where appropriate instead of false. All callers handle this appropriately.

ChangeLog:
	PR ld/32372

	* pdb.c (add_stream): Return NULL.