impl: Strip leading slash from filename if prefix is supplied

If prefix is supplied but does not include a trailing slash,
then the filename will have a leading slash which will cause
int_of_string to fail when trying to interpret the filename
as sequence number later.

Signed-off-by: Euan Harris <euan.harris@citrix.com>
Merge pull request #9 from euanh/master

impl: Strip leading slash from filename if prefix is supplied