Fix condition in Kernel#warn when using uplevel

* It causes SEGV on `warn("foo", uplevel: 100)`.
* Found in a ruby/spec added by @andrykonchin in
  https://github.com/ruby/spec/pull/605

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63650 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

error.c

@@ -371,7 +371,7 @@ rb_warn_m(int argc, VALUE *argv, VALUE exc)
 		args[0] = LONG2NUM(lev + 1);
 		args[1] = INT2FIX(1);
 		location = rb_vm_thread_backtrace_locations(2, args, GET_THREAD()->self);
-		if (!NIL_P(uplevel)) {
+		if (!NIL_P(location)) {
 		    location = rb_ary_entry(location, 0);
 		}
 	    }