sprintf() and strtoul() are massive overkill for turning a string of hex

sprintf() and strtoul() are massive overkill for turning a string of hex
digits into a string of characters; use g_ascii_xdigit_value() to
convert individual hex digit characters to their hex value - and check
whether it succeeds or not, so we catch invalid hex digits! - and just
append the resulting byte value to the string.

Handle the case where compute_ascii_key() fails.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@25265 f5534014-38df-0310-8fa8-9805f1628bb7