Add scripts/coccinelle/deref_null.cocci

Add a Coccinelle file to identify the dereferences of NULL variables

This semantic patch identifies when a variable is known to be NULL
after a test, but it is still dereferenced later.

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