Fix crash under SI95 with multiple receive threads

Fix crash under SI95 with multiple receive threads

When the user application was using multiple threads to receive
messages it was alsmost certain to crash with a segfault.
This change corrects a bug in the ring handler which was
allowing a premature release of the ring lock.

A verification cookie has been added to the msg buffer to
facilitate validation of a message buffer when examining
crash output.

Issue-ID: RIC-218

Signed-off-by: E. Scott Daniels <daniels@research.att.com>
Change-Id: Iaa6519e415dcf7f8c52312e8e02d8cf4dca5866f