windows: Fix control-flow error in select_result().

When making select_result() return void (a2fb1d9), I removed a "return"
at the end of the FD_CLOSE case, causing a fallthrough into FD_ACCEPT
with hilarious (segfaulting) consequences.  Re-instate the "return".

windows/winnet.c

@@ -1765,6 +1765,7 @@ void select_result(WPARAM wParam, LPARAM lParam)
 		    plug_closing(s->plug, NULL, 0, 0);
 	    }
 	} while (ret > 0);
+	return;
        case FD_ACCEPT:
 	{
 #ifdef NO_IPV6