libgo/runtime: return 0, not NULL, from main

Change-Id: I5f905bf31ac7a418a38f0f5817d65f72851120ac
Reviewed-on: https://go-review.googlesource.com/13421
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: axw

libgo/runtime/go-main.c

@@ -38,7 +38,7 @@ main (int argc, char **argv)
   runtime_isarchive = false;
 
   if (runtime_isstarted)
-    return NULL;
+    return 0;
   runtime_isstarted = true;
 
   runtime_check ();