vcomp/lavc: warn if requesting codecs w/o enc

Warn if requesting input encoder pixel formats but the encoder is
not specified explicitly, eg. for
`uv -c lavc:codec=H.264 --param lavc-use-codec=help`
we cannot know, whinch encoder will be used in the
end (usually libx264 for YCbCr and libx264rgb for RGB pixel formats).

src/video_compress/libavcodec.cpp

@@ -456,6 +456,11 @@ void
 handle_help(bool full, string const &req_encoder, string const &req_codec)
 {
         if (req_codec == "help") {
+                if (req_encoder.empty()) {
+                        MSG(ERROR, "The encoder needs to be specified "
+                                   "explictly for input codec listing!\n");
+                        return;
+                }
                 const auto *codec =
                     avcodec_find_encoder_by_name(req_encoder.c_str());
                 if (codec != nullptr) {