Fix use of uninitialized value in checks/files.pm.

When running Lintian from master on a changes files that is not in the
current directory, Lintian emits the following error:

  Use of uninitialized value $1 in concatenation (.) or string at /lcl/lechner/lintian/git/checks/files.pm line 429

It can be reproduced on amd64 with a current master of Lintian by
running in the Lintian root (here called 'git'):

  cd ..
  dget http://deb.debian.org/debian/pool/main/m/mdadm/mdadm_4.1-2.dsc
  cd mdadm-4.1
  debuild
  ../git/frontend/lintian ../mdadm_4.1-2_amd64.changes

This commit checks for null before using the value.

Gbp-Dch: ignore

Signed-off-by: Chris Lamb <lamby@debian.org>
4 jobs for master in 89 minutes and 59 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #251864
stable-bpo

01:26:57

passed #251863
testing

01:25:55

passed #251862
unstable

01:29:59

 
  External
passed jenkins

00:16:16