Dpkg::Index: Check existence of search criteria

When checking a regex or a string match we should avoid comparing
against undefined fields, as those produce perl warnings, and will
never match anyway.

We leave the CODE references alone, as the caller might want to check
if the field value is undefined.

Closes: #780906
Based-on-patch-by: Daniel Dehennin <daniel.dehennin@baby-gnu.org>

debian/changelog

@@ -50,6 +50,9 @@ dpkg (1.18.5) UNRELEASED; urgency=medium
     - Only warn once when a diff patches a file multiple times in
       Dpkg::Source::Patch, and fix the warning message to make it clear that
       the diff might be patchig the file more than once, not just twice.
+    - Check existence of search criteria in Dpkg::Index when checking with a
+      regex or a string match. Closes: #780906
+      Base on a patch by Daniel Dehennin <daniel.dehennin@baby-gnu.org>.
   * Build system:
     - Fix building development documentation.
   * Test suite:

scripts/Dpkg/Index.pm

@@ -227,7 +227,8 @@ sub get_keys {
     foreach my $s_crit (keys %crit) { # search criteria
 	if (ref($crit{$s_crit}) eq 'Regexp') {
 	    @selected = grep {
-		$self->{items}{$_}{$s_crit} =~ $crit{$s_crit}
+		exists $self->{items}{$_}{$s_crit} and
+		       $self->{items}{$_}{$s_crit} =~ $crit{$s_crit}
 	    } @selected;
 	} elsif (ref($crit{$s_crit}) eq 'CODE') {
 	    @selected = grep {
@@ -235,7 +236,8 @@ sub get_keys {
 	    } @selected;
 	} else {
 	    @selected = grep {
-		$self->{items}{$_}{$s_crit} eq $crit{$s_crit}
+		exists $self->{items}{$_}{$s_crit} and
+		       $self->{items}{$_}{$s_crit} eq $crit{$s_crit}
 	    } @selected;
 	}
     }