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/ReplyBotModule.cpp

@@ -168,6 +168,8 @@ void ReplyBotModule::sendDm(const meshtastic_MeshPacket &rx, const char *text)
     if (!text)
         return;
     meshtastic_MeshPacket *p = allocDataPacket();
+    if (!p)
+        return;
     p->to = rx.from;
     p->channel = rx.channel;
     p->want_ack = false;