uclient-http: fix extra compiler warnings on mips_24kc and cortex-a9+neon

uclient-http: fix extra compiler warnings on mips_24kc and cortex-a9+neon

Fixes following warnings as reported on 32-bit platforms
toolchain-mips_24kc_gcc-8.4.0_musl and toolchain-arm_cortex-a9+neon_gcc-8.4.0_musl_eabi:

 uclient-http.c:1111:10: error: comparison of integer expressions of different signedness: 'unsigned int' and 'int' [-Werror=sign-compare]
   if (len > data_end - data)
           ^
 uclient-http.c:1115:11: error: comparison of integer expressions of different signedness: 'unsigned int' and 'long int' [-Werror=sign-compare]
    if (len > uh->read_chunked)
            ^
 uclient-http.c:1120:11: error: comparison of integer expressions of different signedness: 'unsigned int' and 'long int' [-Werror=sign-compare]
    if (len > uh->content_length)
            ^

References: https://gitlab.com/ynezz/openwrt-uclient/-/pipelines/226912126
Signed-off-by: Petr Štetiar <ynezz@true.cz>