avmux: avoid to use unintialized variable

Without this fix, running the command below will get an error randomly.

gst-launch-1.0 videotestsrc ! vp9enc ! avmux_ivf ! fakesink

ERROR: pipeline doesn't want to preroll.
0:00:02.388528491 30148 0x5601b424a370 ERROR                  libav :0::
Tag [1]V[0][0] incompatible with output codec id '167' (VP90)
26 jobs for !67 with fix-avmux in 31 minutes and 49 seconds (queued for 1 second)
merge request