Make partition-lock-release coding more transparent in BufferAlloc().

Coverity complained that oldPartitionLock was possibly dereferenced after
having been set to NULL.  That actually can't happen, because we'd only use
it if (oldFlags & BM_TAG_VALID) is true.  But nonetheless Coverity is
justified in complaining, because at line 1275 we actually overwrite
oldFlags, and then still expect its BM_TAG_VALID bit to be a safe guide to
whether to release the oldPartitionLock.  Thus, the code would be incorrect
if someone else had changed the buffer's BM_TAG_VALID flag meanwhile.
That should not happen, since we hold pin on the buffer throughout this
sequence, but it's starting to look like a rather shaky chain of logic.
And there's no need for such assumptions, because we can simply replace
the (oldFlags & BM_TAG_VALID) tests with (oldPartitionLock != NULL),
which has identical results and makes it plain to all comers that we don't
dereference a null pointer.  A small side benefit is that the range of
liveness of oldFlags is greatly reduced, possibly allowing the compiler
to save a register.

This is just cleanup, not an actual bug fix, so there seems no need
for a back-patch.

Author: tglsfdc

src/backend/storage/buffer/bufmgr.c

@@ -1198,9 +1198,10 @@ BufferAlloc(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 		{
 			/* if it wasn't valid, we need only the new partition */
 			LWLockAcquire(newPartitionLock, LW_EXCLUSIVE);
-			/* these just keep the compiler quiet about uninit variables */
+			/* remember we have no old-partition lock or tag */
+			oldPartitionLock = NULL;
+			/* this just keeps the compiler quiet about uninit variables */
 			oldHash = 0;
-			oldPartitionLock = 0;
 		}
 
 		/*
@@ -1223,7 +1224,7 @@ BufferAlloc(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 			UnpinBuffer(buf, true);
 
 			/* Can give up that buffer's mapping partition lock now */
-			if ((oldFlags & BM_TAG_VALID) &&
+			if (oldPartitionLock != NULL &&
 				oldPartitionLock != newPartitionLock)
 				LWLockRelease(oldPartitionLock);
 
@@ -1277,7 +1278,7 @@ BufferAlloc(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 
 		UnlockBufHdr(buf, buf_state);
 		BufTableDelete(&newTag, newHash);
-		if ((oldFlags & BM_TAG_VALID) &&
+		if (oldPartitionLock != NULL &&
 			oldPartitionLock != newPartitionLock)
 			LWLockRelease(oldPartitionLock);
 		LWLockRelease(newPartitionLock);
@@ -1303,7 +1304,7 @@ BufferAlloc(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 
 	UnlockBufHdr(buf, buf_state);
 
-	if (oldFlags & BM_TAG_VALID)
+	if (oldPartitionLock != NULL)
 	{
 		BufTableDelete(&oldTag, oldHash);
 		if (oldPartitionLock != newPartitionLock)