avfilter/vf_signalstats: fix avframe leak on error

Fixes CID1257014
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/vf_signalstats.c

@@ -132,8 +132,10 @@ static AVFrame *alloc_frame(enum AVPixelFormat pixfmt, int w, int h)
     frame->width  = w;
     frame->height = h;
 
-    if (av_frame_get_buffer(frame, 32) < 0)
+    if (av_frame_get_buffer(frame, 32) < 0) {
+        av_frame_free(&frame);
         return NULL;
+    }
 
     return frame;
 }