fix bug: was writing off end of array

R=gri
OCL=14728
CL=14728
diff --git a/src/syscall/errstr_darwin.go b/src/syscall/errstr_darwin.go
index 8f060c5..65c5255 100644
--- a/src/syscall/errstr_darwin.go
+++ b/src/syscall/errstr_darwin.go
@@ -217,7 +217,6 @@
 	error[EPROTO] = "Protocol error"; 
 	error[ETIME] = "STREAM ioctl timeout"; 
 	error[EOPNOTSUPP] = "Operation not supported on socket"; 
-	error[ELAST] = "Must be equal largest errno"; 
 }
 
 func str(val int64) string {  // do it here rather than with fmt to avoid dependency