Dpkg::Control::Info: Do not print on undef filehandle in output()

Closes: #781887
Signed-off-by: Guillem Jover <guillem@debian.org>

debian/changelog

@@ -88,6 +88,8 @@ dpkg (1.18.0) UNRELEASED; urgency=low
     - Accept an %opts argument for the Dpkg::Control::Info constructor, and
       accept either passing a filename option as undef, or a scalar undef.
       Closes: #782019
+    - Do not print on undef filehandle in Dpkg::Control::Info output().
+      Thanks to Roderich Schupp <roderich.schupp@gmail.com>. Closes: #781887
   * Test suite:
     - Check perl code compilation, warnings and strictness.
     - Fix dpkg-divert unit test to work on BSD «rm -rf» that cannot traverse

scripts/Dpkg/Control/Info.pm

@@ -193,7 +193,7 @@ sub output {
     my $str;
     $str .= $self->{source}->output($fh);
     foreach my $pkg (@{$self->{packages}}) {
-	print { $fh } "\n";
+	print { $fh } "\n" if defined $fh;
 	$str .= "\n" . $pkg->output($fh);
     }
     return $str;