Bugfix: do not die when no layers are detected at all

lib/Slic3r/Print.pm

@@ -129,7 +129,7 @@ sub new_from_mesh {
     }
     
     # remove empty layers from bottom
-    while (!@{$print->layers->[0]->slices}) {
+    while (@{$print->layers} && !@{$print->layers->[0]->slices}) {
         shift @{$print->layers};
         for (my $i = 0; $i <= $#{$print->layers}; $i++) {
             $print->layers->[$i]->id($i);