comment explaining XOR for 8 byte overflow test

test/simple-memory-corruption/overflow_small_8_byte.c

@@ -8,6 +8,7 @@ OPTNONE int main(void) {
         return 1;
     }
     size_t size = malloc_usable_size(p);
+    // XOR is used to avoid the test having a 1/256 chance to fail
     *(p + size + 7) ^= 1;
     free(p);
     return 0;