px5g: creates certificates that expire in the past

the attached patch fixes a bug of px5g when instructed to build
certificates that expire after 2038-01-19, caused a multiplication that
may overflow the "to" variable of type time_t

Attached patch checks if "to" precedes "from": if so sets "to" to its
maximum value. Pretty rude, but works well even if certificate is set to
expire in a century

Signed-off-by: Federico Fissore <federico@fissore.org>
Patchork: http://patchwork.openwrt.org/patch/3749/

git-svn-id: svn://svn.openwrt.org/openwrt@37165 3c298f89-4303-0410-b956-a3cf2f4a3e73