ANDROID: Incremental fs: Fix crash polling 0 size read_log

When read log is 0 sized, we still need to init the wait queue to avoid
kernel panics if someone does decide to poll on the read log.

Test: Added test for this condition, incfs_test crashes
	With fix, incfs_test doesn't crash
Signed-off-by: Paul Lawrence <paullawrence@google.com>
Bug: 152909243
Change-Id: Ic3250523bb7ddb1839f8e95852c17103e5ffb782