segfault fix

Originally committed as revision 4430 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/imgconvert.c

@@ -576,6 +576,8 @@ static void img_copy_plane(uint8_t *dst, int dst_wrap,
                            const uint8_t *src, int src_wrap,
                            int width, int height)
 {
+    if((!dst) || (!src)) 
+        return;
     for(;height > 0; height--) {
         memcpy(dst, src, width);
         dst += dst_wrap;