fix: validate dynamic endpoint index in bridge sample

Reject invalid indices and null device entries to prevent a null-pointer
dereference while handling attribute reads.

Signed-off-by: Dominik Michalczyk <dominik.michalczyk@nordicsemi.no>

Author: dominik-michalczyk-nordic

samples/bridge/src/core/bridge_manager.cpp

@@ -381,10 +381,13 @@ CHIP_ERROR BridgeManager::HandleRead(uint16_t index, ClusterId clusterId,
 				     uint16_t maxReadLength)
 {
 	VerifyOrReturnError(attributeMetadata && buffer, CHIP_ERROR_INVALID_ARGUMENT);
+	VerifyOrReturnError(index != kEmberInvalidEndpointIndex, CHIP_ERROR_INVALID_ARGUMENT);
 	VerifyOrReturnValue(Instance().mDevicesMap.Contains(index), CHIP_ERROR_INTERNAL);
 
 	auto *device = Instance().mDevicesMap[index].mDevice;
 
+	VerifyOrReturnError(device != nullptr, CHIP_ERROR_INTERNAL);
+
 	/* Handle reads for the generic information for all bridged devices. Provide a valid answer even if device state
 	 * is unreachable. */
 	switch (clusterId) {