draw: fix undefined input handling some more...

draw: fix undefined input handling some more...

Previous fixes were incomplete - some code still iterated through the number
of elements provided by velem layout instead of the number stored in the key
(which is the same as the number defined by the vs). And also actually
accessed the elements from the layout directly instead of those in the key.
This mismatch could still cause crashes.
(Besides, it is a very good idea to only use data stored in the key anyway.)
v2: move null format check, remove now unnecessary function parameter,
some minor prettify

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>