avmux: fix segfault when a plugin's long_name is NULL
Some plugins register an empty long_name field. Check for this before calling strcmp to avoid a crash.
A very similar issue was previously fixed in !51 (merged). This patch applies a similar fix.
Issue was encountered in a linux environment:
- gstreamer 1.16.2
- gst-libav 1.16.2
- gst-plugins-bad 1.16.2
- gst-plugins-base 1.16.2
- gst-plugins-good 1.16.2
- ffmpeg 4.2.2