uhttpd: update to latest version, fixes #13564, #13560, improves error handling as pointed out in #13537

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt@36696 3c298f89-4303-0410-b956-a3cf2f4a3e73