Cygwin: Fix warning about address known to be non-NULL in /proc/locales

Fix a gcc 12 warning about an address known to be non-NULL in
format_proc_locale_proc().

> ../../../../src/winsup/cygwin/fhandler/proc.cc: In function ‘BOOL format_proc_locale_proc(LPWSTR, DWORD, LPARAM)’:
> ../../../../src/winsup/cygwin/fhandler/proc.cc:2156:11: error: the address of ‘iso15924’ will never be NULL [-Werror=address]
>  2156 |       if (iso15924)
>       |           ^~~~~~~~
> ../../../../src/winsup/cygwin/fhandler/proc.cc:2133:11: note: ‘iso15924’ declared here
>  2133 |   wchar_t iso15924[ENCODING_LEN + 1] = { 0 };
>       |           ^~~~~~~~

Fixes: c42b98b ("Cygwin: introduce /proc/codesets and /proc/locales")
Signed-off-by: Jon Turney <jon.turney@dronecode.org.uk>

winsup/cygwin/fhandler/proc.cc

@@ -2153,8 +2153,7 @@ format_proc_locale_proc (LPWSTR win_locale, DWORD info, LPARAM param)
       if (!(cp2 = wcschr (cp + 2, L'-')))
         return TRUE;
       /* Otherwise, store in iso15924 */
-      if (iso15924)
-        wcpcpy (wcpncpy (iso15924, cp, cp2 - cp), L";");
+      wcpcpy (wcpncpy (iso15924, cp, cp2 - cp), L";");
     }
   cp = wcsrchr (win_locale, L'-');
   if (cp)