Add volatile modifier to a variable used in the query cache module.

Add volatile modifier to a variable used in the query cache module.

"sts" variable used in pool_fetch_memory_cache() did not have volatile
modifier although it is used in PG_TRY() block. If an exception arises
the value set to variable will be lost in the PG_CATCH block. The bad
effect of this will not be triggered unless error occurs and I think
the bad effect has rarely been observed in the wild (as for as I know,
I have never heard such a report). Anyway, bug is bug.