lib: mempool - Centralize p_free() NULL pointer check

This changes the behavior of p_free(pool, some_null_pointer) slightly.

datastack mempools:

    Previously, the datastack frame id was checked regardless of whether or
    not the pointer was NULL.  Now, only non-NULL pointers perform this
    check.

system mempools:

    Previously, the process would SIGSEGV if a NULL pointer was freed in a
    debug binary on a system with malloc_usable_size().  Now, no SIGSEGV
    occurs.

allocfree, alloconly, and unsafe datastack:

    No change in behavior.

src/lib/mempool-allocfree.c

@@ -267,8 +267,6 @@ static void pool_allocfree_free(pool_t pool, void *mem)
 {
 	struct allocfree_pool *apool =
 		container_of(pool, struct allocfree_pool, pool);
-	if (mem == NULL)
-		return;
 	struct pool_block *block = pool_block_detach(apool, mem);
 	if (apool->clean_frees)
 		safe_memset(block, 0, SIZEOF_POOLBLOCK+block->size);

src/lib/mempool.h

@@ -130,7 +130,8 @@ p_realloc(pool_t pool, void *mem, size_t old_size, size_t new_size)
 
 static inline void p_free_internal(pool_t pool, void *mem)
 {
-	pool->v->free(pool, mem);
+	if (mem != NULL)
+		pool->v->free(pool, mem);
 }
 
 static inline void p_clear(pool_t pool)