Add missing ReleaseBuffer call in BRIN revmap code

I think this particular branch is actually dead, but the analysis to
prove that is not trivial, so instead take the weasel way.

Reported by Jinyu Zhang
Backpatch to 9.5, where BRIN was introduced.

Author: alvherre

src/backend/access/brin/brin_revmap.c

@@ -432,6 +432,7 @@ revmap_physical_extend(BrinRevmap *revmap)
 			if (needLock)
 				UnlockRelationForExtension(irel, ExclusiveLock);
 			LockBuffer(revmap->rm_metaBuf, BUFFER_LOCK_UNLOCK);
+			ReleaseBuffer(buf);
 			return;
 		}
 		LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);