Fix memory leak in new GUC check_hook

Fix memory leak in new GUC check_hook

Commit 38e0190ced71 forgot to pfree() an allocation (freed in other
places of the same function) in only one of several spots in
check_log_min_messages().  Per Coverity.  Add that.

While at it, avoid open-coding guc_strdup().  The new coding does a
strlen() that wasn't there before, but I doubt it's measurable.