runtime: fix integer overflow

The problem happens when end=0, then end-1 is very big number.
Observed with the new scheduler.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/7307073

src/pkg/runtime/parfor.c

@@ -145,7 +145,7 @@ runtime·parfordo(ParFor *desc)
 				// See if it has any work.
 				begin = (uint32)pos;
 				end = (uint32)(pos>>32);
-				if(begin >= end-1) {
+				if(begin+1 >= end) {
 					begin = end = 0;
 					break;
 				}