Bugfix: warnings when trying to process bridges with no anchors

lib/Slic3r/Layer.pm

@@ -341,7 +341,7 @@ sub process_bridges {
                     my $line = Slic3r::Line->new($edges[0]->[0], $edges[0]->[-1]);
                     $bridge_angle = rad2deg_dir($line->direction);
                 }
-            } else {
+            } elsif (@edges) {
                 my $center = bounding_box_center([ map @$_, @edges ]);
                 my $x = my $y = 0;
                 foreach my $point (map @$, @edges) {