on2avc: limit number of bits to 30 in get_egolomb

More don't fit into the integer output.

Also use get_bits_long, since get_bits only supports reading up to 25
bits, while get_bits_long supports the full integer range.

Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Signed-off-by: Anton Khirnov <anton@khirnov.net>