Fix plain return from main() in tests/test-async-sig.c

test-async-sig.c: In function 'main':
test-async-sig.c:185:4: warning: 'return' with no value, in function returning non-void [-Wreturn-type]
diff --git a/tests/test-async-sig.c b/tests/test-async-sig.c
index 296d58d..ba37c90 100644
--- a/tests/test-async-sig.c
+++ b/tests/test-async-sig.c
@@ -182,7 +182,7 @@
       do_backtrace (0, (i++ % 100) == 0);
       if (nerrors > nerrors_max)
         {
-	  panic ("Too many errors (%d)\n", nerrors);
+	  fprintf (stderr, "Too many errors (%d)\n", nerrors);
 	  exit (-1);
         }
     }