bugfix: do not call seq_monitor in pvAssign but reset dbch->monid [amended for branch-2-2]

The problem was a call seq_monitor(ch, FALSE) inside pvAssign after we
destroy the channel (if one exists). This will always fail (strange I
never noticed that) and as a result dbch->monid would not be reset to
NULL. That in turn means when the channel is re-assigned and connects,
seq_monitor(ch, TRUE) will not do anything because it thinks there is
already a valid subscription.

The fix is to remove the call and instead only reset dbch->monid to NULL.

src/seq/seq_if.c

@@ -626,10 +626,13 @@ epicsShareFunc pvStat seq_pvAssign(SS_ID ss, VAR_ID varId, const char *pvName)
 			dbch->connected = FALSE;
 			sp->connectCount--;
 
-			if (ch->monitored)
-			{
-				seq_camonitor(ch, FALSE);
-			}
+			/* Must not call seq_camonitor(ch, FALSE), it would give an
+			error because channel is already dead. pvVarDestroy takes
+                        care that the monid inside the pvid gets invalidated. */
+
+			/* Note ch->monitored remains on because it is a configuration
+			value that belongs to the variable and newly created channels
+			for the same variable should inherit this configuration. */
 		}
 
 		if (status != pvStatOK)