FIJISettingsCanvas: update LFSR vector only if LFSR_WIDTH seems (somewhat) legal

bin/Tk/FIJISettingsCanvas.pm

@@ -457,7 +457,7 @@ sub update {
     # FIUS
     #---------------------------------------------------------------------------
 
-    $self->itemconfigure($fiu_elements->{'0'}->{'lfsrtxt'}, -text => "LFSR [" . ($design_ref->{'LFSR_WIDTH'} - 1) . ":0]");
+    $self->itemconfigure($fiu_elements->{'0'}->{'lfsrtxt'}, -text => "LFSR [" . ($design_ref->{'LFSR_WIDTH'} - 1) . ":0]") if ($design_ref->{'LFSR_WIDTH'} =~ /^[1-9][0-9]*$/);
 
     # Make all FIU-related elements invisible
     for my $i (keys(%{$fiu_elements})) {