locale.c: Move setting errno out of critical section

I doesn't matter much, but its better practice to make critical sections
as small as possible.

locale.c

@@ -4691,8 +4691,8 @@ Perl_mbtowc_(pTHX_ const wchar_t * pwc, const char * s, const Size_t len)
 
 #  else
 
-        MBTOWC_LOCK_;
         SETERRNO(0, 0);
+        MBTOWC_LOCK_;
         retval = mbtowc(NULL, NULL, 0);
         MBTOWC_UNLOCK_;
         return retval;
@@ -4710,8 +4710,8 @@ Perl_mbtowc_(pTHX_ const wchar_t * pwc, const char * s, const Size_t len)
 
     /* Locking prevents races, but locales can be switched out without locking,
      * so this isn't a cure all */
-    MBTOWC_LOCK_;
     SETERRNO(0, 0);
+    MBTOWC_LOCK_;
     retval = mbtowc((wchar_t *) pwc, s, len);
     MBTOWC_UNLOCK_;