Bug 1454659. Remove remaining assert for empty blob images.

When we removed the empty blob image assert in bug 1451458. We ended
up just moving it to this assert. We'll drop this one as well.

gfx/webrender_bindings/src/moz2d_renderer.rs

@@ -421,7 +421,6 @@ impl BlobImageRenderer for Moz2dImageRenderer {
         }
         {
             let mut index = BlobReader::new(&commands);
-            assert!(index.reader.pos < index.reader.buf.len());
             while index.reader.pos < index.reader.buf.len() {
                 let e  = index.read_entry();
                 process_fonts(BufReader::new(&commands[e.end..e.extra_end]), resources);