Null-check packet allocations in allocForSending and its callers (#11086)

* Null-check packet allocations in allocForSending and its callers

Sibling of 0ae44d701.

* Null-check allocDataProtobuf, allocAckNak and allocErrorResponse callers

Second tier of the same nullable contract.

* Keep telemetry sleep scheduling on allocation failure

Allocation failure now marks the telemetry invalid instead of returning
early, so power-saving SENSOR nodes still arm deep sleep.

src/modules/RoutingModule.cpp

@@ -51,6 +51,8 @@ void RoutingModule::sendAckNak(meshtastic_Routing_Error err, NodeNum to, PacketI
                                bool ackWantsAck)
 {
     auto p = allocAckNak(err, to, idFrom, chIndex, hopLimit);
+    if (!p)
+        return;
 
     // Allow the caller to set want_ack on this ACK packet if it's important that the ACK be delivered reliably
     p->want_ack = ackWantsAck;