fix off by one in the socket filename
Review Request #129955 - Created Feb. 16, 2017 and submitted
from the snprintf man page:
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.
pam_wallet.c should increase the needed parameter by one, to account for the terminating null byte ('\0')