reject fuzz inputs with invisible characters

fuzz/fuzz.rs

@@ -41,6 +41,10 @@ pub unsafe extern "C" fn LLVMFuzzerTestOneInput(input: *const u8, len: usize) ->
         Err(_) => return -1,
     };
 
+    if !str.chars().all(|c| c.is_ascii_graphic()) {
+        return -1;
+    }
+
     let atom = str.trim();
 
     let mut stdin = PY_PROCESS.stdin.lock().expect("failed to get stdin lock");