qt_inIsoNametoLCID: protect against a nullptr name

The only user of the function, QCollatorPrivate::init(), passes
QLocalePrivate::bcp47Name().constData(). bcp47Name() may return a
default-constructed QByteArray (e.g. for QLocale::AnyLanguage), so
constData() may be nullptr (QT5_NULL_STRINGS != 1). Passing nullptr to
strncmp() or strncpy() is UB, though.

Instead of using the nullptr-hardened q... versions of these
functions, check name for nullptr once, at the top of the function,
and avoid all the lookup code that follows and is known to fail
(because windows_to_iso_list does not contain empty entries).

This way, we take advantage of the std functions' UB for performance
reasons (fewer repeated nullptr checks), instead of being taken
advantage of.

Qt 5 is not affected, as constData() never returns nullptr there.

Pick-to: 6.5 6.4 6.2
Change-Id: I980dace2bca1e983ac526e89fadeb92239ab5f11
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/text/qlocale_win.cpp

@@ -1038,6 +1038,8 @@ static const char *winLangCodeToIsoName(int code)
 
 LCID qt_inIsoNametoLCID(const char *name)
 {
+    if (!name)
+        return LOCALE_USER_DEFAULT;
     // handle norwegian manually, the list above will fail
     if (!strncmp(name, "nb", 2))
         return 0x0414;