Fix query cancellation.

In commit fe0a0b59, the datatype used for MyCancelKey and other variables
that store cancel keys were changed from long to uint32, but I missed this
one. That broke query cancellation on platforms where long is wider than 32
bits.

Report by Andres Freund, fix by Michael Paquier.

src/backend/postmaster/postmaster.c

@@ -2216,7 +2216,7 @@ processCancelRequest(Port *port, void *pkt)
 {
 	CancelRequestPacket *canc = (CancelRequestPacket *) pkt;
 	int			backendPID;
-	long		cancelAuthCode;
+	int32		cancelAuthCode;
 	Backend    *bp;
 
 #ifndef EXEC_BACKEND
@@ -2226,7 +2226,7 @@ processCancelRequest(Port *port, void *pkt)
 #endif
 
 	backendPID = (int) ntohl(canc->backendPID);
-	cancelAuthCode = (long) ntohl(canc->cancelAuthCode);
+	cancelAuthCode = (int32) ntohl(canc->cancelAuthCode);
 
 	/*
 	 * See if we have a matching backend.  In the EXEC_BACKEND case, we can no