Filesystem: avoid crashes on exit in case the locale codec is null

On exit, QLocale::codecForLocale() can return null as the codec may have
already been destroyed. In that case, pretend that Latin1 was the
locale, so any file name is acceptable. This matches QString:

    QTextCodec *codec = QTextCodec::codecForLocale();
    if (codec)
        return codec->toUnicode(str, size);
 #endif // textcodec
    return fromLatin1(str, size);

Note that if we're wrong and the locale was *not* Latin1, files that you
had a name to may not be encoded or decoded the same way.

Fixes: QTBUG-78446
Change-Id: Iece6e011237e4ab284ecfffd15c54077728a17ca
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
(cherry picked from commit 70e6e9fe590590f602dee230a64870365d9301aa)

src/corelib/io/qfilesystemiterator_unix.cpp

@@ -69,7 +69,9 @@ static bool checkNameDecodable(const char *d_name, qsizetype len)
 #  ifdef QT_LOCALE_IS_UTF8
     int mibEnum = 106;
 #  else
-    int mibEnum = codec->mibEnum();
+    int mibEnum = 4;                // Latin 1
+    if (codec)
+        mibEnum = codec->mibEnum();
 #  endif
     if (Q_LIKELY(mibEnum == 106))   // UTF-8
         return QUtf8::isValidUtf8(d_name, len).isValidUtf8;