sdl/consumer_sdl_audio.c: use mutex when changing refresh_count

sdl/consumer_sdl_audio.c: use mutex when changing refresh_count

Fixes Coverity CID 709359: Data race condition (MISSING_LOCK)

Locking "self->refresh_mutex".
145                pthread_mutex_lock( &self->refresh_mutex );
consumer_sdl_s.refresh_count is being accessed with lock "self->refresh_mutex" held.
146                self->refresh_count = self->refresh_count <= 0 ? 1 : self->refresh_count + 1;
147                pthread_cond_broadcast( &self->refresh_cond );
148                pthread_mutex_unlock( &self->refresh_mutex );
...
Accessing variable "self->refresh_count" (consumer_sdl_s.refresh_count) requires the consumer_sdl_s.refresh_mutex lock.
478        self->refresh_count = 0;
...
Locking "self->refresh_mutex".
535                                pthread_mutex_lock( &self->refresh_mutex );
536                                if ( refresh == 0 && self->refresh_count <= 0 )
537                                {
538                                        consumer_play_video( self, frame );
539                                        pthread_cond_wait( &self->refresh_cond, &self->refresh_mutex );
540                                }
541                                mlt_frame_close( frame );
consumer_sdl_s.refresh_count is being accessed with lock "self->refresh_mutex" held.
542                                self->refresh_count --;
543                                pthread_mutex_unlock( &self->refresh_mutex );