I suspect this overflow check isn't needed at all, at least today, since
vm_map_insert() will detect wraparound when it creates segments (and
even a check against UINT_MAX is too loose, since the max user address
is AOUT32_USRSTACK == 0xbfc00000). But if we're going to keep this
check, there doesn't seem to be any downside to applying it on all
platforms, before exec_new_vmspace() tears down the current vmspace.

Reported by: Muhammed Sariyildiz <asiyee994@gmail.com>