Coccinelle: Move deref_null.cocci to a more appropriate directory

deref_null.cocci is moved to the 'null' directory
which contains other null related rules.

Signed-off-by: Nicolas Palix <npalix@diku.dk>
Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: Michal Marek <mmarek@suse.cz>