#30 un-hex-escape array index error

[kevin montuori]

when un-hex-escape (decode.cl) is called to decode cookie values an
array index error is generated if the value ends with a percent sign:

(un-hex-escape "100%" nil :external-format external-format)

while i'm not sure this is a legal cookie value, it would be preferable if un-
hex-escape handled it gracefully. cheers!