bug#51877: 27.2; term: error in process filter

[Christopher Howard]

Here is the debugger output:

```
Debugger entered--Lisp error: (args-out-of-range "l \3\220\33\22no\4\216s$" 0 
-52)
  substring("l \3\220\33\22no\4\216s$" 0 -52)
  term-emulate-terminal(#<process terminal> 
"l\0l\234\276\217\2\214#\3\342n\4\f\f\204\fd\344\f#|\217\202\237\354\22\223r\222#\204\fc\204\373on\276\0$no\334\342\344\4b\34p...")
```

This is with the encoding set to raw-text and with the terminal set to char 
mode.