backend/ipp.c: Fix printing jobs with long names on older IPP printers

On older printers (ones which don't support IPP operation Create-Job) we concatenate job number and title into one string, which we use as IPP attribute job-name. If the original title was almost 255 chars, the joining the strings will overflow maximal required length for this attribute, and Validate-Job fails.

We could check whether the string is longer than 255 and cut it, but I chose to shrink the buffer to 256, since we already use snprintf() which will cut the string and put null terminator for us.

Fixes #644