Prevent crash on exit.

Calling pthread_cancel( NULL ) results in the same crash as calling
pthread_join( NULL, ... )

This fixes the following:

  Start obecli.
  obecli> set input decklink
  obecli> set input opts card-idx=0
  obecli> probe input
  Probing device: Decklink card 0. Timeout 20 seconds
  Detected input streams:
  Input-stream-id: 0 - Video: RAW 720x576i 25/1fps
  Input-stream-id: 1 - Audio: PCM 16 channels 48kHz
  Encoder outputs:
  Output-stream-id: 0 - Input-stream-id: 0 - Video: AVC
  Output-stream-id: 1 - Input-stream-id: 1 - Audio: RAW - SDI audio pair: 1
  obecli> quit
  closing obe
  Program received signal SIGSEGV, Segmentation fault.
  0x00007ffff6675be1 in pthread_cancel () from /lib64/libpthread.so.0
  (gdb) bt
  #0  0x00007ffff6675be1 in pthread_cancel () from /lib64/libpthread.so.0
  ob-encoder#1  0x00000000004483d1 in obe_close (h=0x1417860) at obe.c:1233
  ob-encoder#2  0x000000000044433f in stop_encode (command=command@entry=0x0, child=child@entry=0x0) at obecli.c:1353
  ob-encoder#3  0x0000000000446d92 in main (argc=<optimized out>, argv=<optimized out>) at obecli.c:1534

obe.c

@@ -559,6 +559,13 @@ static int obe_validate_input_params( obe_input_t *input_device )
     return 0;
 }
 
+static int __pthread_cancel( pthread_t thread )
+{
+    if ( thread )
+        return pthread_cancel( thread );
+    return -1;
+}
+
 static int __pthread_join( pthread_t thread, void **retval )
 {
     if ( thread )
@@ -665,7 +672,7 @@ int obe_probe_device( obe_t *h, obe_input_t *input_device, obe_input_program_t *
             break;
     }
 
-    pthread_cancel( thread );
+    __pthread_cancel( thread );
     __pthread_join( thread, &ret_ptr );
 
     cur_devices = h->num_devices;
@@ -1230,7 +1237,7 @@ void obe_close( obe_t *h )
     /* Cancel input thread */
     for( int i = 0; i < h->num_devices; i++ )
     {
-        pthread_cancel( h->devices[i]->device_thread );
+        __pthread_cancel( h->devices[i]->device_thread );
         __pthread_join( h->devices[i]->device_thread, &ret_ptr );
     }
 
@@ -1304,7 +1311,7 @@ void obe_close( obe_t *h )
         pthread_mutex_unlock( &h->outputs[i]->queue.mutex );
         __pthread_join( h->outputs[i]->output_thread, &ret_ptr );
         /* could be blocking on OS so have to cancel thread too */
-        pthread_cancel( h->outputs[i]->output_thread );
+        __pthread_cancel( h->outputs[i]->output_thread );
         __pthread_join( h->outputs[i]->output_thread, &ret_ptr );
     }