Fix a compilation warning on i386 with clang

src/test/test_util.c

@@ -6299,7 +6299,7 @@ test_util_log_mallinfo(void *arg)
   tt_assert(next2);
   if (mem2 == 0) {
     /* This is a fake mallinfo that doesn't actually fill in its outputs. */
-    tt_int_op(mem1, OP_EQ, 0);
+    tt_u64_op(mem1, OP_EQ, 0);
   } else {
     tt_u64_op(mem1, OP_LT, mem2);
   }