smoothstreamingenc: explict cast to avoid overflow

CC: libav-stable@libav.org
Bug-Id: CID 732248
flvdec: avoid unitialized use of a struct member

CC: libav-stable@libav.org
Bug-Id: CID 718141
flvdec: make sure to check create_stream and report the same error

CC: libav-stable@libav.org
Bug-Id: CID 732242
rmdec: check av_new_packet return value

CC: libav-stable@libav.org
Bug-Id: CID 733714
rmdec: stricter error check to avoid theoretical unitialized use

CC: libav-stable@libav.org
Bug-Id: CID 90558
nutenc: check for negative index rather than assert

CC: libav-stable@libav.org
Bug-Id: CID 703721
nutdec: check av_new_packet return value

CC: libav-stable@libav.org
Bug-Id: CID 733713