We need to set ms_stereo in encode_init() in order to avoid incorrectly
encoding the first frame as non-m/s while flagging it as m/s. Fixes an
uncomfortable pop in the left channel at the start of playback.
CC:libav-stable@libav.org
(cherry picked from commit 51ddf35c9017018e58c15275ff5b129647a0c94d)
Signed-off-by: Reinhard Tartler <siretart@tauware.de>