fix for segv if topic has not been published

If the topic has not been published, orb_copy returns a
negative number which causes update() to memset the data
contents to zero.

In some instances data is a null pointer. This causes a
segment violation crash.

Added a check for data != 0

Signed-off-by: Mark Charlebois <charlebm@gmail.com>

src/modules/mavlink/mavlink_orb_subscription.cpp

@@ -86,8 +86,10 @@ MavlinkOrbSubscription::update(uint64_t *time, void* data)
 	}
 
 	if (orb_copy(_topic, _fd, data)) {
-		/* error copying topic data */
-		memset(data, 0, _topic->o_size);
+		if (data) {
+			/* error copying topic data */
+			memset(data, 0, _topic->o_size);
+		}
 		return false;
 
 	} else {