libs/sgi-uhttpd: do not assume env.CONTENT_LENGTH to be a number

env.CONTENT_LENGTH as passed by uhttpd might be a string, so explicitely
convert it using tonumber().

Issue has been reported by Catalin Patulea <cat@vv.carleton.ca>

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

git-svn-id: http://svn.luci.subsignal.org/luci/trunk@9962 ab181a69-ba2e-0410-a84d-ff88ab4c47bc