index-pack: terminate object buffers with NUL

We have some tricky checks in fsck that rely on a side effect of
require_end_of_header(), and would otherwise easily run outside
non-NUL-terminated buffers. This is a bit brittle, so let's make sure
that only NUL-terminated buffers are passed around to begin with.

Jeff "Peff" King contributed the detailed analysis which call paths are
involved and pointed out that we also have to patch the get_data()
function in unpack-objects.c, which is what Johannes "Dscho" Schindelin
implemented.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Analyzed-by: Jeff King <peff@peff.net>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/index-pack.c

@@ -438,7 +438,7 @@ static void *unpack_entry_data(unsigned long offset, unsigned long size,
 	if (type == OBJ_BLOB && size > big_file_threshold)
 		buf = fixed_buf;
 	else
-		buf = xmalloc(size);
+		buf = xmallocz(size);
 
 	memset(&stream, 0, sizeof(stream));
 	git_inflate_init(&stream);
@@ -543,7 +543,7 @@ static void *unpack_data(struct object_entry *obj,
 	git_zstream stream;
 	int status;
 
-	data = xmalloc(consume ? 64*1024 : obj->size);
+	data = xmallocz(consume ? 64*1024 : obj->size);
 	inbuf = xmalloc((len < 64*1024) ? len : 64*1024);
 
 	memset(&stream, 0, sizeof(stream));

builtin/unpack-objects.c

@@ -91,7 +91,7 @@ static void use(int bytes)
 static void *get_data(unsigned long size)
 {
 	git_zstream stream;
-	void *buf = xmalloc(size);
+	void *buf = xmallocz(size);
 
 	memset(&stream, 0, sizeof(stream));