Coccinelle: Add iterators/itnull.cocci

Many iterators have the property that the first argument is always bound
to a real list element, never NULL.  False positives arise for some
iterators that do not have this property, or in cases when the loop
cursor is reassigned.  The latter should only happen when the matched
code is on the way to a loop exit (break, goto, or return).

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