x264enc: fix crash on NULL caps when negotiation happens without a downstream peer

gst-launch-1.0 videotestsrc ! x264enc
x264enc: fix compiler warning

Compiler warns rightly about possibly uninitialized variable.
mpeg2dec: Use new gst_video_decoder_set_needs_format() API
amrwbdec: Use gst_audio_decoder_set_needs_format() instead of custom code
amrnbdec: Use gst_audio_decoder_set_needs_format() instead of custom code
amrnbdec: Remove some dead code
amrwbdec: Remove some dead code