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: NNicolas Palix <npalix@diku.dk>
Signed-off-by: NJulia Lawall <julia@diku.dk>
Signed-off-by: NMichal Marek <mmarek@suse.cz>