avformat/utils: Check av_packet_new_side_data() return before using it

Fixes null pointer dereference
Fixes CID1108592
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avutil/file_open: Print debug message if setting close on exec fails

Fixes CID1087079
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avutil/opt: initialize ret

Fixes CID1108610
Fixes use of uninitialized variable

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