fix bug187 not to crash

TBR=austin
DELTA=9  (4 added, 4 deleted, 1 changed)
OCL=33115
CL=33115

test/bugs/bug187.go

@@ -6,13 +6,17 @@
 
 package main
 
+import "os"
+
 func main() {
 	// This bug doesn't arise with [...]int, or []interface{} or [3]interface{}.
 	a := [...]interface{} { 1, 2, 3 };
 	n := 0;
+	bug := false;
 	for _, v := range a {
 		if v.(int) != n {
-			panicln("BUG:", n, v.(int));
+			println("BUG:", n, v.(int));
+			os.Exit(0);
 		}
 		n++;
 	}

test/golden.out

@@ -156,7 +156,3 @@ BUG: errchk: command succeeded unexpectedly
 
 =========== bugs/bug187.go
 BUG: 0 1
-SIGSEGV: segmentation violation
-Faulting address: 0x40a1f0
-PC=0x40a1f0
-