Make failmalloc disable itself on program exit.

Make failmalloc disable itself on program exit.

Something in the Travis build environment has changed recently to cause
failmalloc runs to crash while exiting due to a NULL pointer dereference
in the coverage writing code. This fix causes failmalloc to
automatically disable itself once main() exits so subsequent memory
requests always succeed. Since the application (and library) have no
control over what happens during exit cleanup anyway, this does not
affect its functionality.