sms: Added result buffer size parameter to 7bit conv funs

The 7bit<->8bit encoding/decoding functions didn't check whether
there is still enough space in the destination buffer. Therefore a
buffer size parameter has been added to each of the functions which
is used to truncate the output if the buffer is too small.

In addition, the return value of the decoding functions has been
changed to number of characters written (excluding \0), so this
value is always equal to strlen(decoded).

The old functions are still available as wrapper functions.
8 files changed