lavu: fix GET_UTF8 macro.

Prevent 0xFE and 0xFF from being considered the start of
non-standard 7-bytes sequences. No noticeable effect on speed.
lavc: check decoded subtitles encoding.

Address trac ticket #2431.
Merge remote-tracking branch 'cigaes/master'

* cigaes/master:
  lavc: check decoded subtitles encoding.
  lavu: fix GET_UTF8 macro.

Merged-by: Michael Niedermayer <michaelni@gmx.at>