fix pkt_normalizer: null deref on alloc fail, mtu underflow, unchecked pn_buf_renew in loop

src/pkt_normalizer.c

@@ -30,7 +30,6 @@ struct PKTNORM* pn_init(struct ETCP_CONN* etcp) {
 
     struct PKTNORM* pn = u_calloc(1, sizeof(struct PKTNORM));
     if (!pn) {
-        pn->alloc_errors++;
         DEBUG_ERROR(DEBUG_CATEGORY_NORMALIZER, "pn_init: calloc failed");
         return NULL;
     }
@@ -40,7 +39,12 @@ struct PKTNORM* pn_init(struct ETCP_CONN* etcp) {
 
     pn->etcp = etcp;
     pn->ua = etcp->instance->ua;
-    pn->frag_size = etcp->mtu - ACK_REZERV - UDP_HDR_SIZE - UDP_SC_HDR_SIZE;  // Use MTU as fixed packet size (adjust if headers need subtraction)
+    pn->frag_size = etcp->mtu - ACK_REZERV - UDP_HDR_SIZE - UDP_SC_HDR_SIZE;
+    int min_frag = UDP_HDR_SIZE + UDP_SC_HDR_SIZE + ACK_REZERV;
+    if (etcp->mtu < min_frag || pn->frag_size > etcp->mtu) {
+        DEBUG_ERROR(DEBUG_CATEGORY_NORMALIZER, "pn_init: MTU %d too small (min %d)", etcp->mtu, min_frag);
+        pn->frag_size = etcp->mtu;
+    }
     pn->tx_wait_time = 10;
 
     pn->input = queue_new(pn->ua, 0, 0, 0, "pn_input");  // No hash needed
@@ -298,6 +302,7 @@ static void etcp_input_ready_cb(struct ll_queue* q, void* arg) {
         pn->data_ptr += remain;
         in_ptr += remain;
         pn_buf_renew(pn);
+        if (!pn->data) goto exit;
     }
 
 exit: