unpack-trees: make sure "df_conflict_entry.name" is NUL terminated.

unpack-trees: make sure "df_conflict_entry.name" is NUL terminated.

The structure that ends with a flexible array member (or 0
length array with older GCC) "char name[FLEX_ARRAY]" is
allocated on the stack and we use it after clearing its entire
size with memset.  That does not guarantee that "name" is
properly NUL terminated as we intended on platforms with more
forgiving structure alignment requirements.

Reported breakage on m68k by Roman Zippel.

Signed-off-by: Junio C Hamano <junkio@cox.net>