ovs-thread: Fix OVS_ADAPTIVE_MUTEX_INITIALIZER.

Commit 05bf6d3c62e1d (ovs-thread: Add checking for mutex and rwlock
initialization.) updated mutex and rwlock initializers to set the "where"
member to a nonnull value, but missed this one.  This commit fixes the
problem.

This does not cause real problems in practice because
OVS_ADAPTIVE_MUTEX_INITIALIZER has no existing users.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>