Update unity_fixture.c

[../Unity-master/extras/fixture/src/unity_fixture.c:210]: (error) Memory leak: guard

Found by https://github.com/bryongloden/cppcheck

extras/fixture/src/unity_fixture.c

@@ -207,6 +207,7 @@ void* unity_malloc(size_t size)
     mem = (char*)&(guard[1]);
     memcpy(&mem[size], end, sizeof(end));
 
+    free(guard);
     return (void*)mem;
 }