nl: avoid NULL pointer dereference

nl: avoid NULL pointer dereference

It's a valid case to call nla_put() with NULL data and 0 len. It's done e.g. in
the nla_put_attr().

There has to be a check for data in nla_put() as passing NULL to the memcpy()
is not allowed. Even if length is 0, both pointers have to be valid.

For a reference see C99 standard (7.21.1/2), it says: "pointer arguments on
such a call shall still have valid values".

Reported-by: Daniel Gimpelevich <daniel@gimpelevich.san-francisco.ca.us>
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
[christian.brauner@ubuntu.com: adapted commit message]
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>