get_rtc_time, in the case that PDC returns that the battery is bad, returns
an unmodified rtc_time arg to the caller, which then uses uninitialized
values. Fix this by memset-ing the arg with zeroes, so it will at least be
cleared if we return failure.

Spotted by John David Anglin.
Signed-off-by: Kyle McMartin <kyle@mcmartin.ca>