avmux: avoid to use unintialized variable
Without this fix, running the command below will get an error randomly.
Example: 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)