libc/strptime: fix warning unused-value

warning: value computed is not used [-Wunused-value]
Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>
Signed-off-by: anjiahao <anjiahao@xiaomi.com>

libs/libc/time/lib_strptime.c

@@ -413,8 +413,8 @@ _conv_num(const unsigned char **buf, int *dest, int llim, int ulim)
     int result = 0;
     int rulim = ulim;
 
-    while (ispace(**buf))
-        *buf++;
+    while (isspace(**buf))
+        (*buf)++;
 
     if (**buf < '0' || **buf > '9')
         return (0);