osd_libass: increase robustness when handling internal OSD escapes

The \xFF escape is used internally to insert special OSD symbols (which
need a font change to the internal OSD font). There was potential for
breakage when \xFF was followed by \0, because then "in" would be
advanced past the string's end.

Normally this can't happen, as it would require invalid UTF-8 input
data. But we don't check input for UTF-8 validness, so there's a
potential issue here. Garbled output is ok on invalid UTF-8 input,
but crashing is not.

Make it more robust by checking for this.

sub/osd_libass.c

@@ -234,7 +234,7 @@ static char *mangle_ass(const char *in)
     char *res = talloc_strdup(NULL, "");
     while (*in) {
         // As used by osd_get_function_sym().
-        if (in[0] == '\xFF') {
+        if (in[0] == '\xFF' && in[1]) {
             res = talloc_strdup_append_buffer(res, ASS_USE_OSD_FONT);
             res = append_utf8_buffer(res, OSD_CODEPOINTS + in[1]);
             res = talloc_strdup_append_buffer(res, "{\\r}");