charset_conv: fallback to interpreting subs as latin1 if iconv fails

For display purposes, it's better to show scrambled text - at least
that's a more actionable failure mode than spamming the terminal with
FFmpeg nonsense error messages.

This avoids the obnoxious and pointless

  "Invalid UTF-8 in decoded subtitles text; maybe missing -sub_charenc option"

FFmpeg error, which will be spammed on every single subtitle event. We
don't even have a -sub-charenc option, fuck FFmpeg.

Did I mention fuck FFmpeg yet? Because fuck FFmpeg.

misc/charset_conv.c

@@ -234,5 +234,5 @@ bstr mp_iconv_to_utf8(struct mp_log *log, bstr buf, const char *cp, int flags)
 #endif
 
 failure:
-    return (bstr){0};
+    return bstr_sanitize_utf8_latin1(NULL, buf);
 }