uhttpd: Fix multiple format string problems

uhttpd: Fix multiple format string problems

After format string checks were activated in libubox the compiler
started to complain about multiple missuses in uhttpd. This fixes the
format strings without changing the behavior.

blobmsg_get_string() just checks if the parameter is not NULL and then
calls blobmsg_data() and casts the result.

I think non of these problem is security relevant.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>