gck-slot: Initialize struct tm to all-zeroes (#42)

If the format string for strptime() doesn't include a time zone, then the tm_isdst member will be left uninitialized (see NOTES in Linux strptime(3)). This means we will be off by an hour from the intended time if whatever arbitrary junk is on the stack happens to include a positive value for tm.tm_isdst.

Resolves: #42 (closed)
Bug-Debian: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=953981