Fix _lookup_serial to compare with proper types.

Storing a serial number in a long and comparing it to an ogg_uint32_t only
 works if you cast the long down, instead of letting C promote it.


git-svn-id: https://svn.xiph.org/trunk/Tremor@17536 0101bb08-14d6-0310-b084-bc0e0c8e3800