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>

libs/sgi-uhttpd/luasrc/sgi/uhttpd.lua

@@ -43,7 +43,7 @@ function handle_request(env)
 		renv["HTTP_" .. k] = v
 	end
 
-	local len = env.CONTENT_LENGTH or 0
+	local len = tonumber(env.CONTENT_LENGTH) or 0
 	local function recv()
 		if len > 0 then
 			local rlen, rbuf = uhttpd.recv(4096)