str-format: Fixed format_hex_string_with_delimiter.

g_snprintf adds a terminating NULL character, which was not considered.
We need more space for this extra character, so the result length
check should also be tweaked.

Signed-off-by: Viktor Tusa <tusa@balabit.hu>
Merge pull request #78 from talien/f/format-hex-string-fix

str-format: Fixed format_hex_string_with_delimiter.