host/logmux: Fix nil cursor panic when reading empty log
host/logmux: Fix nil cursor panic when reading empty log

Signed-off-by: Jonathan Rudenberg <3692bfa45759a67d83aedf0045f6cb635a966abf@titanous.com>

flynn/flynn,flynn/flynn,flynn/flynn,lmars/flynn,flynn/flynn,lmars/flynn,lmars/flynn,lmars/flynn,lmars/flynn,flynn/flynn