theora: Skip zero-sized headers

This fixes a regression since d9cf5f51/7a2ee770f5 with theora
over RTP (possibly with other variants of theora as well).

In theora over RTP, the second of the 3 headers turns out to be
0 bytes long, which prior to d9cf5f51 worked just fine. After
d9cf5f51, reading from the bitstream reader fails (since the reader
wasn't initialized but returned an error if initialized with 0 bits).

CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>
(cherry picked from commit e33db35b4a91ad543d9dde3a981a89118ba68937)

Signed-off-by: Martin Storsjö <martin@martin.st>