Fix warning on repository without md5sum

A repository without MD5sum will cause warnings:

Use of uninitialized value $lines{"MD5sum:"} in concatenation (.) or
string at /usr/bin/apt-mirror line 787

The written MD5 file is not used by apt-mirror. Thus skip missing MD5Sum
values.

Example repository without MD5Sum:
deb-amd64 https://packages.chef.io/stable-apt xenial main

fixes #66

1 files changed, 1 insertions, 1 deletions

diff --git a/apt-mirror b/apt-mirror
index ec2b09d..f05aae0 100755
--- a/apt-mirror
+++ b/apt-mirror
@@ -813,7 +813,7 @@ sub process_index
         {    # Packages index
             $skipclean{ remove_double_slashes( $path . "/" . $lines{"Filename:"} ) } = 1;
             print FILES_ALL remove_double_slashes( $path . "/" . $lines{"Filename:"} ) . "\n";
-            print FILES_MD5 $lines{"MD5sum:"} . "  " . remove_double_slashes( $path . "/" . $lines{"Filename:"} ) . "\n";
+            print FILES_MD5 $lines{"MD5sum:"} . "  " . remove_double_slashes( $path . "/" . $lines{"Filename:"} ) . "\n" if defined $lines{"MD5sum:"};
             if ( need_update( $mirror . "/" . $lines{"Filename:"}, $lines{"Size:"} ) )
             {
                 print FILES_NEW remove_double_slashes( $uri . "/" . $lines{"Filename:"} ) . "\n";