af_channelmap: fix ONE_STR mapping mode

get_channel() returns 0 on success

CC:libav-stable@libav.org
FATE: add a test for the ONE_STR mapping mode of the channelmap filter
Merge commit 'e843612695007cc623813073754c651ab43021f7'

* commit 'e843612695007cc623813073754c651ab43021f7':
  af_channelmap: fix ONE_STR mapping mode

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

* qatar/master:
  FATE: add a test for the ONE_STR mapping mode of the channelmap filter

Conflicts:
	tests/fate/filter-audio.mak

Merged-by: Michael Niedermayer <michaelni@gmx.at>
ffmpeg_opt: check that a subtitle encoder is available before auto mapping streams

Fixes Ticket3470

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