Byte string `length` incorrect

[letmaik]

Adding the following to one of the unit tests shows the issue:

len = unhex("581868656c6c6f6f6f6f6f6f6f6f6f6f6f6f6f6f206e75727365",start);
// ...
fail_unless(memcmp(res->start+res->header,"helloooooooooooooo nurse",res->length) == 0);
fail_unless(res->length == 24); // actual: 25

PR #1 fixes this issue and should be merged.