Bad checksum messages ending with MAVLINK_STX causing parser get out of sync

[tomerg-flytrex]

I found an issue where if the c parser gets a message that does not pass the checksum check, and the second checksum byte is MAVLINK_STX the parser will get out of sync and won't be able to parse the next message, possibly more than that depending on their content.
The issue is with the mavlink_parse_char function in mavlink_helpers.h. The code there checks if the second checksum byte is MAVLINK_STX and if it is then it counts it as the start of the next message. I stumbled this issue when testing out the PingRx ADS-B with ArduCopter, which for some reason sends messages not defined in the uAvionix xml. This caused the parser to fail parsing them due to the crc_extra being wrong, which in turn sometimes leads to 20 seconds without successfully parsed messages. Obviously adding the missing messages to the xml could solve it in this case but I don't see the reason behind this check:

	    if (c == MAVLINK_STX)
	    {
		    status->parse_state = MAVLINK_PARSE_STATE_GOT_STX;
		    rxmsg->len = 0;
		    mavlink_start_checksum(rxmsg);
	    }