reflect: fix Zero() implementation - not every type has a zero object allocated, so we still need to allocate a new zero area every time.
reflect: fix Zero() implementation - not every type has a
zero object allocated, so we still need to allocate a new
zero area every time.

Fixes issue 6876.

R=golang-dev
CC=golang-dev
https://codereview.appspot.com/36320043

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang