server.c: Fix wrong priority for do_exit

Under some specific circumstances, poll(2) would return a positive
integer, but do_exit might had been previously set. This caused libweb
to ignore SIGTERM, with the potential risk for an endless loop.

server.c

@@ -230,13 +230,13 @@ again:
 
     res = poll(fds, n, -1);
 
-    if (res < 0)
+    if (do_exit)
+    {
+        *exit = true;
+        goto end;
+    }
+    else if (res < 0)
     {
-        if (do_exit)
-        {
-            *exit = true;
-            goto end;
-        }
 
         switch (errno)
         {