Fix is_zend_ptr() huge block comparison

We should compare the block memory, not the block metadata (See
zend_mm_add_huge_block).
This caused random test failure for ext/ffi/tests/gh14626.phpt when the
malloc() performed by the FFI code lies close to the block metadata, and
the size of the block is large enough.

This was reported by php#16902 (comment)

Zend/zend_alloc.c

@@ -2457,8 +2457,8 @@ ZEND_API bool is_zend_ptr(const void *ptr)
 
 	zend_mm_huge_list *block = AG(mm_heap)->huge_list;
 	while (block) {
-		if (ptr >= (void*)block
-				&& ptr < (void*)((char*)block + block->size)) {
+		if (ptr >= block->ptr
+				&& ptr < (void*)((char*)block->ptr + block->size)) {
 			return 1;
 		}
 		block = block->next;