avcodec_decode_audio4: check got_frame_ptr before handling initial skip

      Fixes out of array accesses

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 8a6449167a6da8cb747cfe3502ae86ffaac2ed48)

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