codecs: h264decoder: Don't give up to decode due to missing reference picture

Merged Seungha Yang requested to merge seungha.yang/gst-plugins-bad:h264-ref-pic-enhance into master

Missing reference picture is very common thing for broken/malformed stream. Decoder should be able to keep decoding if it's not a very critical error.

Fixes: #1443 (moved)

CC @vjaquez @ndufresne

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