In ERTM, the outgoing data queue contains both unsent data and unacked
data, and sk_send_head points to the first unsent packet. When
disconnecting a socket, it's possible for a process to attempt a
socket send while a disconnection is in progress and the outgoing data
queue has been purged. When this purge happens, sk_send_head must also
be set to NULL because the packet it was pointing to has been freed.
Change-Id: I87b37a13583fac705241549dfc492950c7c0ec0b
CRs-fixed: 326238
Signed-off-by: Mat Martineau <mathewm@codeaurora.org>