Don't potentially dereference NULL in net/dcb/dcbnl.c:dcbnl_getapp()

nla_nest_start() may return NULL. If it does then we'll blow up in
nla_nest_end() when we dereference the pointer.

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: David S. Miller <davem@davemloft.net>