dns: Fix dns_alloc_pcb for reuse an existing one case

The logic to use an already existing pcb is wrong because the idx never
advanced in the for loop, so it keep checking the same dns_pcbs[idx] for
each loop iteration. Fix it.

Signed-off-by: Axel Lin <axel.lin@ingics.com>

src/core/dns.c

@@ -851,8 +851,7 @@ dns_alloc_pcb(void)
   }
   /* if we come here, creating a new UDP pcb failed, so we have to use
      an already existing one */
-  idx = dns_last_pcb_idx + 1;
-  for (i = 0; i < DNS_MAX_SOURCE_PORTS; i++) {
+  for (i = 0, idx = dns_last_pcb_idx + 1; i < DNS_MAX_SOURCE_PORTS; i++, idx++) {
     if (idx >= DNS_MAX_SOURCE_PORTS) {
       idx = 0;
     }