openh264: Fail gracefully if openh264 encoder/decoder creation fails

This can happen with the dummy "noopenh264" library that the freedesktop flatpak runtime ships, and Fedora is planning on shipping as well. In both cases the dummy implementation gets replaced with the actual openh264 library that's downloaded directly from Cisco, but just to be on safe side, this patch makes it careful to check the return values to avoid crashing if the underlying library hasn't been swapped out yet.

The patch is taken from freedesktop-sdk and was originally written by Valentin David valentin.david@codethink.co.uk.