wtv: Seek by sector properly

Use an helper function to seek by sector to avoid possible mistakes
due shifting by WTV_SECTOR_BITS a 32bit integer.

Contrary to common intuition, a 32 bit integer left shifted
by a 64 bit integer does not promote the 32 bit integer to
64 bit before shifting.
avformat/wtv: rename to wtv_common

This helps git matchup files on merges correctly

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Merge remote-tracking branch 'qatar/master'

* qatar/master:
  wtv: Seek by sector properly

Conflicts:
	libavformat/wtvdec.c

See: a0d13d84a928110b0e99ea16d17fd758fcbde35f
Merged-by: Michael Niedermayer <michaelni@gmx.at>