Remove unnecessary NULL test.

Spotted by Coverity and reported by Michael Paquier.  Per discussion,
we don't necessarily care about making Coverity happy in all such
instances, but we can go ahead and change them where it otherwise
seems to improve the code.

src/bin/pg_upgrade/controldata.c

@@ -402,7 +402,6 @@ get_control_data(ClusterInfo *cluster, bool live_check)
 		}
 	}
 
-	if (output)
 	pclose(output);
 
 	/*