Setting a frame title that contains non-Unicode characters causes a crash in the NS backend. (Other platforms may or may not deal with it appropriately -- if you have the opportunity to test, please report.)

Since the title is typically derived from the buffer name, this is easily reproduced by

 (rename-buffer "n\351")

The crash occurs in ns_set_name_internal:

  encoded_name = ENCODE_UTF_8 (name);

Here encoded_name is still "n\351" (a 2 byte unibyte string), because the \351 couldn't be encoded.

  str = [NSString stringWithUTF8String: SSDATA (encoded_name)];

Now str is nil since "n\351" isn't valid UTF-8.

    [[view window] setTitle: str];

Here we get an NS crash because nil isn't a valid setTitle: argument.

Proposed patch attached. I didn't find any obvious way to encode an Emacs string into valid UTF-8 (with bad parts replaced) so a new function was written. The corresponding Lisp function was marked internal because it's only there for test purposes, but it could of course be promoted to non-internal if someone wants it.