uORB: fix hardfault in uORB calloc implementation

When running out-of-memory, the malloc returns NULL and the memset then tries to write to address 0 which results in a hardfault.

platforms/common/uORB/uORBDeviceNode.cpp

@@ -188,8 +188,11 @@ uORB::DeviceNode::write(cdev::file_t *filp, const char *buffer, size_t buflen)
 			if (nullptr == _data) {
 				const size_t data_size = _meta->o_size * _queue_size;
 				_data = (uint8_t *) px4_cache_aligned_alloc(data_size);
+
+				if (_data) {
 					memset(_data, 0, data_size);
 				}
+			}
 
 			unlock();