avformat/movenc: Check input sample count

      Fixes: division by 0
Fixes: fpe_movenc.c_199_1.wav
Fixes: fpe_movenc.c_199_2.wav
Fixes: fpe_movenc.c_199_3.wav
Fixes: fpe_movenc.c_199_4.wav
Fixes: fpe_movenc.c_199_5.wav
Fixes: fpe_movenc.c_199_6.wav
Fixes: fpe_movenc.c_199_7.wav

Found-by: #CHEN HONGXU# <HCHEN017@e.ntu.edu.sg>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>