fast-import: zero all of 'struct tag' to silence valgrind

fast-import: zero all of 'struct tag' to silence valgrind

When running t9300, valgrind (correctly) complains about an
uninitialized value in write_crash_report:

  ==2971== Use of uninitialised value of size 8
  ==2971==    at 0x4164F4: sha1_to_hex (hex.c:70)
  ==2971==    by 0x4073E4: die_nicely (fast-import.c:468)
  ==2971==    by 0x43284C: die (usage.c:86)
  ==2971==    by 0x40420D: main (fast-import.c:2731)
  ==2971==  Uninitialised value was created by a heap allocation
  ==2971==    at 0x4C29B3D: malloc (vg_replace_malloc.c:263)
  ==2971==    by 0x433645: xmalloc (wrapper.c:35)
  ==2971==    by 0x405DF5: pool_alloc (fast-import.c:619)
  ==2971==    by 0x407755: pool_calloc.constprop.14 (fast-import.c:634)
  ==2971==    by 0x403F33: main (fast-import.c:3324)

Fix this by zeroing all of the 'struct tag'.  We would only need to
zero out the 'sha1' field, but this way seems more future-proof.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>