avformat/oggparseflac: check ff_alloc_extradata() return code

Fixes CID1108573
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/oggparsespeex: Check for extradata allocation failure

Fixes CID1108574
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>