Dpkg::BuildFlags: avoid a Perl warning when HOME is not set

debian/changelog

@@ -32,6 +32,7 @@ dpkg (1.16.1) UNRELEASED; urgency=low
     when dpkg-dev is absent.
   * Check presence of required parameters in dpkg-vendor. Closes: #628726
     Thanks to Niels Thykier <niels@thykier.net> for the patch.
+  * Avoid a Perl warning in dpkg-buildflags when HOME is not set.
 
   [ Guillem Jover ]
   * Install deb-src-control(5) man pages in dpkg-dev. Closes: #620520

scripts/Dpkg/BuildFlags.pm

@@ -106,8 +106,11 @@ Update flags from the user configuration.
 
 sub load_user_config {
     my ($self) = @_;
-    my $confdir = $ENV{'XDG_CONFIG_HOME'} || $ENV{"HOME"} . "/.config";
-    $self->update_from_conffile("$confdir/dpkg/buildflags.conf", "user");
+    my $confdir = $ENV{'XDG_CONFIG_HOME'};
+    $confdir ||= $ENV{'HOME'} . "/.config" if defined $ENV{'HOME'};
+    if (defined $confdir) {
+        $self->update_from_conffile("$confdir/dpkg/buildflags.conf", "user");
+    }
 }
 
 =item $bf->load_environment_config()