clear freed memory in u_free_impl to catch UAF earlier

lib/mem.c

@@ -233,6 +233,8 @@ void u_free_impl(void* ptr, const char* location) {
 #endif
         exit(EXIT_FAILURE);
     }
+    uint32_t total = size + 12 + 2 * BOUNDARY_CHECK_SIZE + METADATA_SIZE;
+    memset(base, 0, total);
     free(base);
 }