parse_time: always initialize the (fallback) display format

This fixes a segfault that can happen when the user supplies an ill-formatted
SLURM_TIME_FORMAT string. Fixed by always initializing the display_fmt string.

Instead of segfaulting, the response now uses the fallback:
01_BUG-Fix_parse_time_segfault.diff from Gerrit Renker, CSCS.