Fix "ignoring extra $criterion" errors.

This is a very old bug that had been perplexing me for years.  Of course, the
solution is trivial, but tracking it down involved hours of debugging using
some rather convoluted code.

Thanks, TPF, for affording me the opportunity find and fix this.

lib/Devel/Cover/DB/Structure.pm

@@ -173,7 +173,7 @@ sub set_subroutine
         # print STDERR "new $file:$line:$sub_name\n";
         $self->{count}{$_}{$file} =
             $self->{f}{$file}{start}{$line}{$sub_name}[$scount]{$_} =
-            $self->get_count($_)
+            $self->get_count($file, $_)
             for $self->criteria;
     }
     # print STDERR "set_subroutine start $file:$line $sub_name($scount) ",
@@ -186,7 +186,7 @@ sub store_counts
     my ($file) = @_;
     $self->{count}{$_}{$file} =
         $self->{f}{$file}{start}{-1}{__COVER__}[0]{$_} =
-        $self->get_count($_)
+        $self->get_count($file, $_)
         for $self->criteria;
     # print STDERR "store_counts: ", Dumper $self->{f}{$file}{start};
 }
@@ -241,9 +241,8 @@ sub digest
 sub get_count
 {
     my $self = shift;
-    my ($criterion) = @_;
-    return 0 unless $self->{file};  # TODO - how does this get unset?
-    $self->{count}{$criterion}{$self->{file}}
+    my ($file, $criterion) = @_;
+    $self->{count}{$criterion}{$file}
 }
 
 sub add_count