Prevent crash due to access to an already destroyed atomic object

[nanang]

The PJLIB mutex functions all check for NULL mutexes but pj_atomic_destroy() is not setting its mutex to NULL after it destroyed it so any attempts to use the atomic again would cause a crash. This ticket does not address why an attempt was made to use the atomic again but it does prevent the crash.

Thank you George Joseph for the patch.

[nanang]

In 5758:

Close #2101:

• set atomic's mutex to NULL in atomic destroy
• added few sanity checks to the atomic functions.