find-debuginfo-files: Fix processing of root directory

`%dir /' would cause errors (ALT#38842):

  error: No such file or directory: /usr/src/tmp/broken-debug-buildroot/usr/lib/debug/lib/modules
  error: No such file or directory: /usr/src/tmp/broken-debug-buildroot/usr/lib/debug/lib/modules/vmlinux

Reported-by: Vitaly Lipatov <lav@altlinux.org>
BZ: https://bugzilla.altlinux.org/show_bug.cgi?id=38842

scripts/find-debuginfo-files.in

@@ -22,6 +22,8 @@ mkdir .tmp
 while read -r f; do
 	f=${f#$RPM_BUILD_ROOT}
 
+	[ -n "$f" ] || continue
+
 	if [ -z "${f##/boot/vmlinuz-*}" ]; then
 		# Redirect to vmlinux saved by debuginfo.brp into debug tree's
 		# %modules_dir.