Add exception to swift-drive-audit if no date in line.

Sometimes there is no date at the beginning of a line in kern.log.
Although it does not happen often, there should be a check ensuring
the program doesn't crash in case it happens.

Added try-except block surrounding parsing string to date format.

Change-Id: I44a101266582eea2199189a006afa1037a9bd4ea
Fixes: bug #1152658