fix a overflow condition in parse time fn for x32 arch

If the result equals to LONG LONG MAX, there are two possible reasons:

1. overflow
2. time is 9223372036854775807s after 1970 (292278994-08-17)

So, we apologize to all future users affected by this simplification.

Uncovered by coverity.

Related to rhbz#1016571

Signed-off-by: Jakub Filak <jfilak@redhat.com>