fix(mesh): don't assert on malloc() failure in MemoryDynamic::alloc() (#11197)

* fix(mesh): don't assert on malloc() failure in MemoryDynamic::alloc()

MemoryDynamic<T>::alloc() called assert(p) right after malloc(), instead
of returning nullptr like the static MemoryPool<T,N>::alloc() already
does on exhaustion. All of packetPool's callers were already hardened to
null-check allocCopy()/allocZeroed() (#10948, #10951), but that path is
unreachable on real OOM here: assert() fires first, one level down,
before the caller ever gets a chance to check anything.

On STM32WL (packetPool is MemoryDynamic there - not enough static RAM
for the fixed pool), assert() failures are wrapped to an infinite
`while(true);` loop rather than aborting or resetting, so the thread
just hangs forever instead of returning nullptr. Reproduced on wio-e5
hardware under a burst of incoming DMs, with free heap draining toward
zero right before the hang.

Assisted-by: Claude Sonnet 5 <noreply@anthropic.com>
Signed-off-by: Andrew Yong <me@ndoo.sg>

* fix: null-check the two allocUnique*() call sites missed by prior audits

#10948/#10951 null-checked every raw-pointer packetPool.allocCopy()/
allocZeroed() call site, but missed the two spots using the
UniquePacketPoolPacket (unique_ptr) wrapper: UdpMulticastHandler::onReceive()
and MQTT::onReceive() both dereferenced the allocation result unconditionally.

Previously unreachable in practice: MemoryDynamic::alloc() asserted before
ever returning null, so nothing downstream saw it. Fixed alongside that
assert removal (this branch) since these two are now reachable with a
genuine null - same fix as everywhere else, just an unchecked pointer that
turns up under OOM.

Assisted-by: Claude Sonnet 5 <noreply@anthropic.com>
Signed-off-by: Andrew Yong <me@ndoo.sg>

* fix(SimRadio): don't leave isReceiving stuck true on allocation failure

startReceive() set isReceiving = true before allocCopy(), so a failed
allocation (now reachable with a real nullptr instead of hanging in
assert()) left the simulated radio permanently marked as receiving:
handleReceiveInterrupt() returns immediately on a null receivingPacket,
before ever reaching the code that would clear isReceiving.

Assisted-by: Claude Sonnet 5 <noreply@anthropic.com>
Signed-off-by: Andrew Yong <me@ndoo.sg>

---------

Signed-off-by: Andrew Yong <me@ndoo.sg>
Co-authored-by: Ben Meadors <benmmeadors@gmail.com>

src/platform/portduino/SimRadio.cpp

@@ -323,13 +323,19 @@ void SimRadio::startReceive(meshtastic_MeshPacket *p)
             return;
         }
     }
-    isReceiving = true;
     receivingPacket = packetPool.allocCopy(*p);
+    if (!receivingPacket) {
+        return;
+    }
+    isReceiving = true;
     uint32_t airtimeMsec = getPacketTime(p, true);
     notifyLater(airtimeMsec, ISR_RX, false); // Model the time it is busy receiving
 #else
-    isReceiving = true;
     receivingPacket = packetPool.allocCopy(*p);
+    if (!receivingPacket) {
+        return;
+    }
+    isReceiving = true;
     handleReceiveInterrupt(); // Simulate receiving the packet immediately
     startTransmitTimer();
 #endif