Since commit e9b9e4ace2 has landed, there is a chance that
container.RWLayer is nil (due to some half-removed container). Let's
check the pointer before use to avoid any potential nil pointer
dereferences, resulting in a daemon crash.
Note that even without the abovementioned commit, it's better to perform
an extra check (even it's totally redundant) rather than to have a
possibility of a daemon crash. In other words, better be safe than
sorry.
[v2: add a test case for daemon.getInspectData]
[v3: add a check for container.Dead and a special error for the case]
Fixes: e9b9e4ace2
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
