mctp i2c: handle NULL header address

daddr can be NULL if there is no neighbour table entry present,
in that case the tx packet should be dropped.

saddr will usually be set by MCTP core, but check for NULL in case a
packet is transmitted by a different protocol.

Fixes: f5b8abf9 ("mctp i2c: MCTP I2C binding driver")
Cc: stable@vger.kernel.org
Reported-by: Dung Cao <dung@os.amperecomputing.com>
Signed-off-by: Matt Johnston <matt@codeconstruct.com.au>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20241022-mctp-i2c-null-dest-v3-1-e929709956c5@codeconstruct.com.au


Signed-off-by: Jakub Kicinski <kuba@kernel.org>