netlink: Fix for potential NULL-pointer deref

netlink: Fix for potential NULL-pointer deref

If memory allocation fails, calloc() returns NULL which was not checked
for. The code seems to expect zero array size though, so simply
replacing this call by one of the x*calloc() ones won't work. So guard
the call also by a check for 'len'.

Fixes: db0697ce7f602 ("src: support for flowtable listing")
Signed-off-by: Phil Sutter <phil@nwl.cc>