ptx: fix panic caused by improper assert (#10916)

* ptx:  fix panic caused by improper assert

* fix formatting

src/uu/ptx/src/ptx.rs

@@ -501,7 +501,7 @@ fn get_output_chunks(
     // and get the string.
     let before_str: String = all_before[before_beg..before_end].iter().collect();
     before.push_str(&before_str);
-    assert!(max_before_size >= before.len());
+    assert!(max_before_size >= before.chars().count());
 
     // the after chunk