Additional check to return a friendly error when binary STL is not valid

lib/Slic3r/Config.pm

@@ -391,7 +391,7 @@ sub validate {
     }
     
     if ($Slic3r::flow_width >= ($Slic3r::nozzle_diameter + $Slic3r::layer_height)) {
-        # rectangle with shrunk at the ends
+        # rectangle with semicircles at the ends
         $Slic3r::min_flow_spacing = $Slic3r::flow_width - $Slic3r::layer_height * (1 - PI/4);
     } else {
         # rectangle with shrunk semicircles at the ends

lib/Slic3r/STL.pm

@@ -22,6 +22,7 @@ sub read_file {
         seek $fh, 80, 0;
         read $fh, my $buf, 4;
         my $triangle_count = unpack 'L', $buf;
+        die "STL file seems invalid, could not read facet count\n" if !defined $triangle_count;
         my $expected_size =
             + 80 # header
             +  4 # count