BUILD/MINOR: ssl: remove one call to sprintf()

BUILD/MINOR: ssl: remove one call to sprintf()

Lukas reported another OpenBSD complaint about this use of sprintf() that
I missed :

src/ssl_sock.o(.text+0x2a79): In function `bind_parse_crt':
src/ssl_sock.c:3015: warning: sprintf() is often misused, please use snprintf()

This one was even easier to handle. Note that some of these calls could
be simplified by checking the snprintf output size instead of doing the
preliminary size computation.