BUG/MINOR: map/cli: make sure patterns don't vanish under "show map"'s init

BUG/MINOR: map/cli: make sure patterns don't vanish under "show map"'s init

When "show map" initializes itself, it first takes the reference to the
starting point under a lock, then releases it before switching to state
STATE_LIST, and takes the lock again. The problem is that it is possible
for another thread to remove the first element during this unlock/lock
sequence, and make the list run anywhere. This is of course extremely
unlikely but not impossible.

Let's initialize the pointer in the STATE_LIST part under the same lock,
which is simpler and more reliable.

This should be backported to all versions.