demmt: fix processing of ioctl messages on buffer boundary

broken by commit e07efbd3e235952342c7ddb4f6d1494657ea360d
"demmt: pass memory dump messages to ioctl handlers"
demmt: don't print "[no data]" for NULL pointers when decoding ioctl mthd calls
demmt: more ioctl decoding

Note: in order to use it, you need to obtain traces with the latest
mmt version.