fix(router): null-check packetPool/clientNotificationPool allocations (#10948)

On platforms where these pools are heap-backed (MemoryDynamic - used
whenever there isn't enough static RAM for a fixed pool, e.g.
ARCH_STM32WL or BOARD_HAS_PSRAM), allocCopy()/allocZeroed() return
nullptr on allocation failure and already log a warning, but most
callers dereferenced the result unconditionally. Under real heap
pressure this reliably produced a HardFault - reproduced on STM32WL
hardware under sustained mesh traffic, including the RX entry point
(RadioLibInterface::handleReceiveInterrupt) where every received
packet is allocated.

Adds null checks at all call sites that were missing one, mirroring
the guard pattern already used correctly elsewhere in the same files
(e.g. RadioInterface.cpp's sendErrorNotification). On allocation
failure, callers now skip the send/retransmission/notification and
log nothing further (the allocator already did) rather than crash.

Assisted-by: Claude Sonnet 5 <noreply@anthropic.com>

Signed-off-by: Andrew Yong <me@ndoo.sg>

src/mesh/ReliableRouter.cpp

@@ -21,7 +21,8 @@ ErrorCode ReliableRouter::send(meshtastic_MeshPacket *p)
         auto copy = packetPool.allocCopy(*p);
         DEBUG_HEAP_AFTER("ReliableRouter::send", copy);
 
-        startRetransmission(copy, NUM_RELIABLE_RETX);
+        if (copy)
+            startRetransmission(copy, NUM_RELIABLE_RETX);
     }
 
     /* If we have pending retransmissions, add the airtime of this packet to it, because during that time we cannot receive an