Fix possible NULL dereference in ExecAlterObjectDependsStmt

I used the wrong variable here.  Doesn't make a difference today because
the only plausible caller passes a non-NULL variable, but someday it
will be wrong, and even today's correctness is subtle: the caller that
does pass a NULL is never invoked because of object type constraints.
Surely not a condition to rely on.

Noted by Coverity

Author: alvherre

src/backend/commands/alter.c

@@ -422,7 +422,7 @@ ExecAlterObjectDependsStmt(AlterObjectDependsStmt *stmt, ObjectAddress *refAddre
 	if (refAddress)
 		*refAddress = refAddr;
 
-	recordDependencyOn(&address, refAddress, DEPENDENCY_AUTO_EXTENSION);
+	recordDependencyOn(&address, &refAddr, DEPENDENCY_AUTO_EXTENSION);
 
 	return address;
 }