toggle_locale_i: eliminate NULL for %s format warning

The C99 printf() family of functions do not permit a NULL
pointer for the %s format specifier.

In this case, there's a NULL check done immediately afterwards,
allowing the compiler to detect that locale_to_restore_to could
be NULL.

While PerlIO_printf() (via sv_vcatpvfn_flags()) does allow NULL,
it isn't permitted by the standard implementation, so gcc warns.

Ideally the compiler would provide customisation hooks for this
warning, but it doesn't, and since the -Wformat family of
warnings are useful, do the minimal change to prevent it in
this case.

locale.c

@@ -4622,7 +4622,8 @@ S_toggle_locale_i(pTHX_ const locale_category_index cat_index,
                            "Entering toggle_locale_i: index=%d(%s),"        \
                            " wanted=%s, actual=%s; called from %" LINE_Tf   \
                            "\n", cat_index, category_names[cat_index],
-                           new_locale, locale_to_restore_to, caller_line));
+                           new_locale, locale_to_restore_to ? locale_to_restore_to : "(null)",
+                           caller_line));
 
     if (! locale_to_restore_to) {
         locale_panic_via_(Perl_form(aTHX_