Leak when using libevent and pthread

Hello,

I found a leak when the function evthread_use_pthreads is used. This function call evthread_use_pthreads_with_flags and then pthread_mutexattr_init 2 times.

pthread_mutexattr_init seems to allocate 20 bytes that need to be freed with pthread_mutexattr_destroy when not longer used.

This is not a big problem but still a leak.