Fix unused results warnings

Fix unused results warnings

Fixes:

uclient-http.c:385:8: error: ignoring return value of 'fread', declared with attribute warn_unused_result [-Werror=unused-result]
   fread(&val, sizeof(val), 1, f);
        ^

uclient-fetch.c: In function 'main':
uclient-fetch.c:664:12: error: ignoring return value of 'asprintf', declared with attribute warn_unused_result [-Werror=unused-result]
    asprintf(&auth_str, "%s:%s", username, password);
            ^
uclient-fetch.c: In function 'read_data_cb':
uclient-fetch.c:269:9: error: ignoring return value of 'write', declared with attribute warn_unused_result [-Werror=unused-result]
    write(output_fd, buf, len);

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>