avfilter/buffersrc: Use correct, matching deallocation function

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

libavfilter/buffersrc.c

@@ -242,7 +242,7 @@ do {                                                                    \
     ref_out = av_buffer_create(data, data_size, compat_unref_buffer,    \
                                dummy_ref, (buf->perms & AV_PERM_WRITE) ? 0 : AV_BUFFER_FLAG_READONLY);                           \
     if (!ref_out) {                                                     \
-        av_freep(&dummy_ref);                                           \
+        av_buffer_unref(&dummy_ref);                                    \
         av_frame_unref(frame);                                          \
         ret = AVERROR(ENOMEM);                                          \
         goto fail;                                                      \