merge: fix out-of-bounds memory access

merge: fix out-of-bounds memory access

The parameter n of unpack_callback() can have a value of up to
MAX_UNPACK_TREES.  The check at the top of unpack_trees() (its only
(indirect) caller) makes sure it cannot exceed this limit.

unpack_callback() passes it and the array src to unpack_nondirectories(),
which has this loop:

	for (i = 0; i < n; i++) {
		/* ... */
		src[i + o->merge] = o->df_conflict_entry;

o->merge can be 0 or 1, so unpack_nondirectories() potentially accesses
the array src at index MAX_UNPACK_TREES.  This patch makes it big enough.

Reported-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: René Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>