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

    
    Reviewed-on: https://go-review.googlesource.com/13421

From-SVN: r227673

gcc/go/gofrontend/MERGE

-9bac6243d2252b2d043243d6ab1123ba7e90fa53
+352617bfe0a880febf5d2a87e89ea439c742ba18
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

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 ();