Add missing check for wal_debug GUC.

9a20a9b2 added a new elog(), enabled when WAL_DEBUG is defined. The
other WAL_DEBUG dependant messages check for the wal_debug GUC, but this
one did not. While at it replace 'upto' with 'up to'.

Discussion: 20150610110253.GF3832@alap3.anarazel.de

Backpatch to 9.4, the first release containing 9a20a9b2.