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>

unpack-trees.c

@@ -240,8 +240,11 @@ static struct cache_entry *create_ce_entry(const struct traverse_info *info, con
 	return ce;
 }
 
-static int unpack_nondirectories(int n, unsigned long mask, unsigned long dirmask, struct cache_entry *src[5],
-	const struct name_entry *names, const struct traverse_info *info)
+static int unpack_nondirectories(int n, unsigned long mask,
+				 unsigned long dirmask,
+				 struct cache_entry **src,
+				 const struct name_entry *names,
+				 const struct traverse_info *info)
 {
 	int i;
 	struct unpack_trees_options *o = info->data;
@@ -291,7 +294,7 @@ static int unpack_nondirectories(int n, unsigned long mask, unsigned long dirmas
 
 static int unpack_callback(int n, unsigned long mask, unsigned long dirmask, struct name_entry *names, struct traverse_info *info)
 {
-	struct cache_entry *src[5] = { NULL, };
+	struct cache_entry *src[MAX_UNPACK_TREES + 1] = { NULL, };
 	struct unpack_trees_options *o = info->data;
 	const struct name_entry *p = names;