Fix potential bug in test file

[MicroMilo]

All the bugs detected can be checked easily.

first commit

this commit fixes the file descriptor leak.

There exists a potential file descriptor leak in fuzz_main.c, which fopen a file but forget to fclose it.

second commit

this commit fixes the null pointer dereference and memory leak.

In the function cJSON_Print, NULL value may be returned, we need to take care of this case.
What's more, before exit the program, we should free the resource as more as we can.

third commit

this commit fixes the potential memory leak

4th commit

this commit fixes the potential Null Pointer Dereference