Fix wxLocale::IsOk() in case of initialization failure

Ever since 700256bbdb IsOk() returned true even
if setting the locale actually failed because the old locale was still set to
the null value.

Apply the minimal possible fix for this and just reset the old locale pointer
to null if initializing the locale fails to make sure IsOk() doesn't return
true in this case.

docs/changes.txt

@@ -580,6 +580,7 @@ Major new features in this release
 All:
 
 - Fix wxFileName::MakeRelativeTo() for directory relatively to itself.
+- Fix wxLocale::IsOk() return true even if setting the locale failed.
 
 Unix: