xfrm: Fix xfrm_input() to verify state is valid when (encap_type < 0)

Code path when (encap_type < 0) does not verify the state is valid
before progressing.

This will result in a crash if, for instance, x->km.state ==
XFRM_STATE_ACQ.

Fixes: 7785bba2 ("esp: Add a software GRO codepath")
Signed-off-by: Aviv Heller <avivh@mellanox.com>
Signed-off-by: Yevgeny Kliteynik <kliteyn@mellanox.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>