Slightly reduce lock usage

Locking the frame_buffer mutex to reference the input frame into the
tmp_frame is unnecessary.

app/src/frame_buffer.c

@@ -51,8 +51,6 @@ swap_frames(AVFrame **lhs, AVFrame **rhs) {
 bool
 sc_frame_buffer_push(struct sc_frame_buffer *fb, const AVFrame *frame,
                   bool *previous_frame_skipped) {
-    sc_mutex_lock(&fb->mutex);
-
     // Use a temporary frame to preserve pending_frame in case of error.
     // tmp_frame is an empty frame, no need to call av_frame_unref() beforehand.
     int r = av_frame_ref(fb->tmp_frame, frame);
@@ -61,6 +59,8 @@ sc_frame_buffer_push(struct sc_frame_buffer *fb, const AVFrame *frame,
         return false;
     }
 
+    sc_mutex_lock(&fb->mutex);
+
     // Now that av_frame_ref() succeeded, we can replace the previous
     // pending_frame
     swap_frames(&fb->pending_frame, &fb->tmp_frame);