codecparsers: Variant of _identify_nalu without checks

codecparsers: Variant of _identify_nalu without checks

This is useful for cases where the caller *knows* that the provided
input contains a whole NALU and can therefore avoid:
* the expensive checks for the next start code (which won't be present)
* delaying the input parsing (since we would need the next incoming NALU
  in order for the parsing code to detect the next start code)

https://bugzilla.gnome.org/show_bug.cgi?id=665584