fix buffer overflow in debug log (bsc#1207403)

[wfeldt]

Task

Port https://github.com/openSUSE/linuxrc/pull/315 to SLE15-SP5.

Original problem

• https://bugzilla.suse.com/show_bug.cgi?id=1207403

SHA-512 signed repodata lead to very long file names that triggered a buffer overflow.