*.[ch] refactoring: make use of the FREE_AND_NULL() macro

Replace occurrences of `free(ptr); ptr = NULL` which weren't caught by
the coccinelle rule. These fall into two categories:

 - free/NULL assignments one after the other which coccinelle all put
   on one line, which is functionally equivalent code, but very ugly.

 - manually spotted occurrences where the NULL assignment isn't right
   after the free() call.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit-slab.h

@@ -82,8 +82,7 @@ static MAYBE_UNUSED void clear_ ##slabname(struct slabname *s)		\
 	for (i = 0; i < s->slab_count; i++)				\
 		free(s->slab[i]);					\
 	s->slab_count = 0;						\
-	free(s->slab);							\
-	s->slab = NULL;							\
+	FREE_AND_NULL(s->slab);						\
 }									\
 									\
 static MAYBE_UNUSED elemtype *slabname## _at_peek(struct slabname *s,	\