Fix: missing statedump end event when block device listing available

Fix: missing statedump end event when block device listing available

Incorrect handling of lttng_enumerate_block_devices() return value of 0
(which means "ok"), causing do_lttng_statedump() to return early.

To causes missing "lttng_statedump_end" in traces where block device
enumeration is available (CONFIG_KALLSYMS=y).

Reported-by: Antoine Busque <abusque@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>