Initialize 'ra' to zero to avoid uninitialized use.

Initialize 'ra' to zero to avoid uninitialized use.

If the instruction in this case does not include an RA field, then 'ra'
is used uninitialized.  Use the same idiom used elsewhere in this file of
initializing ra to zero before check for an RA field.

gdb/ChangeLog:

	* rs6000-tdep.c (ppc_process_record_op31): Initialize ra.