fix warning if ltime is unsigned

wolfcrypt/src/asn.c

@@ -13777,7 +13777,7 @@ int wc_ValidateDate(const byte* date, byte format, int dateType)
     (void)tmpTime;
 
     ltime = wc_Time(0);
-    if (ltime < 0){
+    if (sizeof(ltime) == sizeof(word32) && (int)ltime < 0){
         /* A negative response here could be due to a 32-bit time_t
          * where the year is 2038 or later. */
         WOLFSSL_MSG("wc_Time failed to return a valid value");