Fix character "@" added at end of received messages

Fix character "@" added at end of received messages

If 7 bit message encoded on 8 bit cells has one last bit occupying
whole byte it need to be padded with 7 zero bits. To not mistake
these last 7 bits with a character with code 0 (@) we need to check
length from User Data Length field and not rely on actual length
of received data. An example can be message "abcdefg" encoded
as "61 f1 98 5c 36 9f 01", it is decoded as "abcdefg@".

Signed-off-by: SÅ‚awomir Demeszko <s.demeszko@wireless-instruments.com>