Merge pull request #38 from XXXXzzzz000/fix_error

修复gcc编译报错

easyflash/src/ef_env_wl.c

@@ -616,7 +616,7 @@ void ef_print_env(void) {
         for (j = 0; j < 4; j++) {
             c = (*env_cache_detail_addr) >> (8 * j);
             ef_print("%c", c);
-            if (c == NULL) {
+            if (c == '\0') {
                 ef_print("\n");
                 break;
             }