ppc: avoid buffer overrun: use pstrcpy, not strncpy

A terminal NUL is required by caller's use of strchr.
It's better not to use strncpy at all, since there is no need
to zero out hundreds of trailing bytes for each iteration.

Signed-off-by: Jim Meyering <meyering@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
