Do not show "Estimating duration from bitrate" warning if no duration was estimated.
Fix compilation with --enable-decoder=webp --disable-decoder=vp8
Merge remote-tracking branch 'cehoyos/master'

* cehoyos/master:
  Fix compilation with --enable-decoder=webp --disable-decoder=vp8
  Do not show "Estimating duration from bitrate" warning if no duration was estimated.

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