debugedit: Fix off-by-one adding DW_FORM_string replacement slashes.

debugedit: Fix off-by-one adding DW_FORM_string replacement slashes.

We would put one too many slashes in between the new dest_dir and file name
part of the replacement of a DW_FORM_string in the .debug_info. If there
was file part then we would overwrite the first character of the name. If
there was no file part at all then this would overwrite the zero terminator
and cause a crash reading the rest of the data.

A crash did happen while building the docker package on fedora s390x.
https://bugzilla.redhat.com/show_bug.cgi?id=1434347

The reason neither issue would normally trigger is because if we do detect
that the dest_dir is larger than the base_dir we refuse to replace anything.

Signed-off-by: Mark Wielaard <mark@klomp.org>