gst-libav: fix Valgrind false-positive memcheck

Fixes: #3203 (closed)

During video decoding, when the selected ffmpeg decoder can use direct rendering, output buffers are allocated by gst-libav and then fulfilled by libavcodec. In this case, Valgrind cannot detect that the memory blocks have been modified and still considers them as non-initialised memory.

Whenever a downstream element tries to read the decoded frame content, Valgrind will issue a "uninitialised value" error. To prevent this false-positive detection, we help Valgrind by explicitly tagging the corresponding memory blocks as initialised.