strftime: Always restore LC_CTYPE locale

my_strftime toggles LC_CTYPE to match LC_TIME to avoid mojibake.

In reading the code, I realized that there was a code path that doesn't
properly restore it, namely when failure is returned.  This commit fixes
that.

locale.c

@@ -7135,7 +7135,7 @@ S_strftime_tm(pTHX_ const char *fmt, const struct tm *mytm)
          * indicates we have at least one byte of spare space (which will be
          * used for the terminating NUL). */
         if (inRANGE(len, 1, bufsize - 1)) {
-            goto strftime_success;
+            goto strftime_return;
         }
 
         /* There are several possible reasons for a 0 return code for a
@@ -7169,7 +7169,7 @@ S_strftime_tm(pTHX_ const char *fmt, const struct tm *mytm)
     if (strEQ(fmt, "%p")) {
         Renew(buf, 1, char);
         *buf = '\0';
-        goto strftime_success;
+        goto strftime_return;
     }
 
     /* The other reason is that the format string is malformed.  Probably it is
@@ -7178,9 +7178,9 @@ S_strftime_tm(pTHX_ const char *fmt, const struct tm *mytm)
      * treated as a literal, but others may fail when '?' is illegal */
     Safefree(buf);
     SET_EINVAL;
-    return NULL;
+    buf = NULL;
 
-  strftime_success:
+  strftime_return:
 
 #  if defined(USE_LOCALE_CTYPE) && defined(USE_LOCALE_TIME)