In transform method, now checks that the transformed exon is the same

length as the original exon, returning undef if not (can happen when
only part of the exon could be transformed)

modules/Bio/EnsEMBL/Exon.pm

@@ -428,7 +428,10 @@ sub transform {
   }
 
   my $new_exon = $self->SUPER::transform( @_ );
-  return undef unless $new_exon;
+  if (not defined $new_exon or
+      $new_exon->length != $self->length) {
+    return undef;
+  }
 
   if( exists $self->{'_supporting_evidence'} ) {
     my @new_features;