gc: fix wrong arguments to error message for switches.

Fixes #2502.

R=golang-dev, rsc
CC=golang-dev, remy
https://golang.org/cl/5472062

src/cmd/gc/swt.c

@@ -878,7 +878,7 @@ typecheckswitch(Node *n)
 						if(n->ntest)
 							yyerror("invalid case %N in switch on %N (mismatched types %T and %T)", ll->n, n->ntest, ll->n->type, t);
 						else
-							yyerror("invalid case %N in switch (mismatched types %T and bool)", ll->n, n->ntest, ll->n->type, t);
+							yyerror("invalid case %N in switch (mismatched types %T and bool)", ll->n, ll->n->type);
 					} else if(nilonly && !isconst(ll->n, CTNIL)) {
 						yyerror("invalid case %N in switch (can only compare %s %N to nil)", ll->n, nilonly, n->ntest);
 					}