mm-sms-part-3gpp: avoid buffer overflow if packed data is too large
With GSM7 encoding, packedlen is the length of the unpacked string after expanding septets to octets so it will be ~14% bigger than the original string length. This means we have to be careful not to copy too much data into the PDU buffer.
Similar issues exist in other branches of the same function.
Fixes #768. Thanks rhezashan@gmail.com for the report.
Edited by Eric Caruso