Assert that strong-lock count is >0 everywhere it's decremented.

The one existing assertion of this type has tripped a few times in the
buildfarm lately, but it's not clear whether the problem is really
originating there or whether it's leftovers from a trip through one
of the other two paths that lack a matching assertion.  So add one.

Since the same bug(s) most likely exist(s) in the back-branches also,
back-patch to 9.2, where the fast-path lock mechanism was added.

src/backend/storage/lmgr/lock.c

@@ -1541,6 +1541,7 @@ AbortStrongLockAcquire(void)
 	fasthashcode = FastPathStrongLockHashPartition(locallock->hashcode);
 	Assert(locallock->holdsStrongLockCount == TRUE);
 	SpinLockAcquire(&FastPathStrongRelationLocks->mutex);
+	Assert(FastPathStrongRelationLocks->count[fasthashcode] > 0);
 	FastPathStrongRelationLocks->count[fasthashcode]--;
 	locallock->holdsStrongLockCount = FALSE;
 	StrongLockInProgress = NULL;
@@ -2953,6 +2954,7 @@ LockRefindAndRelease(LockMethod lockMethodTable, PGPROC *proc,
 		uint32		fasthashcode = FastPathStrongLockHashPartition(hashcode);
 
 		SpinLockAcquire(&FastPathStrongRelationLocks->mutex);
+		Assert(FastPathStrongRelationLocks->count[fasthashcode] > 0);
 		FastPathStrongRelationLocks->count[fasthashcode]--;
 		SpinLockRelease(&FastPathStrongRelationLocks->mutex);
 	}