Fix potential buffer underflow and inefficient copy using fnmatch.

[robert-ancell]

If length was zero we could check the -1 index. A nul was always added because we only used the length of the string, not the buffer (i.e. off by one). Also remove a check for a negative number from an unsigned number.

[hughsie]

CI seems to be failing -- can you check than out pls?

[hughsie]

@robert-ancell ?