avcodec/pthread_frame: fix missing unlock on error

Fixes CID1197057
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/pthread_frame.c

@@ -356,8 +356,10 @@ static int submit_packet(PerThreadContext *p, AVPacket *avpkt)
         memcpy(p->buf, avpkt->data, avpkt->size);
         memset(p->buf + avpkt->size, 0, FF_INPUT_BUFFER_PADDING_SIZE);
     }
-    if ((ret = av_copy_packet_side_data(&p->avpkt, avpkt)) < 0)
+    if ((ret = av_copy_packet_side_data(&p->avpkt, avpkt)) < 0) {
+        pthread_mutex_unlock(&p->mutex);
         return ret;
+    }
 
     p->state = STATE_SETTING_UP;
     pthread_cond_signal(&p->input_cond);