Update unity_fixture.c

Sorry @mvandervoord -- not your code, your repository 😁

Regarding the double free, cppcheck has a check for that too (just saying) 👍

extras/fixture/src/unity_fixture.c

@@ -207,7 +207,7 @@ void* unity_malloc(size_t size)
     mem = (char*)&(guard[1]);
     memcpy(&mem[size], end, sizeof(end));
 
-#ifndef UNITY_EXCLUDE_STDLIB_MALLOC
+#ifndef UNITY_FIXTURE_MALLOC_OVERRIDES_H_
     free(guard);
 #endif
     return (void*)mem;