ext/theora/theoraenc.c: Check return value of theora_encode_header(), or we might try to allocate a random number of ...

Original commit message from CVS:
* ext/theora/theoraenc.c: (theora_enc_chain):
Check return value of theora_encode_header(), or we might try to
allocate a random number of bytes. theora_encode_header() can fail
if libtheora has been compiled with encoding support disabled.
Fixes #398110.