Fix snprintf always return 0

[xchapron-ledger]

Initially reported here: LedgerHQ/nanos-secure-sdk#28 by @shufps

Technically man snprintf says that:

RETURN VALUE
       Upon successful return, these functions return the number of characters printed (excluding the null byte used to end output to strings).

       The functions snprintf() and vsnprintf() do not write more than size bytes (including the terminating null byte ('\0')).  If the output was truncated due to this limit, then the return value is  the
       number  of characters (excluding the terminating null byte) which would have been written to the final string if enough space had been available.  Thus, a return value of size or more means that the
       output was truncated.  (See also below under NOTES.)

       If an output error is encountered, a negative value is returned.

So always returning 0 is not a standard behavior.
However fixing this perfectly will be difficult as it would need to determine the number of char that will overrun the buffer...
I guess a first step would be to:

• return return (uint32_t) str - (uint32_t) op_str; as proposed here snprintf always return 0 nanos-secure-sdk#28 (comment) when the buffer size is enough.
• return size when the output was truncated.