Fixed that the device crashes when users send SMS via STK.

If the user sends an SMS through STK menu, and the SMS encoding
is 7-bit GSM, the device will crash because of buffer overflow.
Fixed by supplying the correct buffer length while doing the
array copy.

bug: 23501570
Change-Id: I569b519a87ae693b0c35371ee160ca52fd5b6e6a