avformat/avidec: Fix infinite loop in avi_read_nikon()

      Fixes: 360/test.poc

Found-by: 连一汉 <lianyihan@360.cn>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit e4e4a9cad7f21593d4bcb1f2404ea0d373c36c43)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>