We can handle the special case of num_stripes == 0 directly inside
btrfs_read_sys_array. The BUG_ON in btrfs_chunk_item_size is there to
catch other unhandled cases where we fail to validate external data.

A crafted or corrupted image crashes at mount time:

BTRFS: device fsid 9006933e-2a9a-44f0-917f-514252aeec2c devid 1 transid 7 /dev/loop0
BTRFS info (device loop0): disk space caching is enabled
BUG: failure at fs/btrfs/ctree.h:337/btrfs_chunk_item_size()!
Kernel panic - not syncing: BUG!
CPU: 0 PID: 313 Comm: mount Not tainted 4.2.5-00657-ge047887-dirty #25
Stack:
 637af890 60062489 602aeb2e 604192ba
 60387961 00000011 637af8a0 6038a835
 637af9c0 6038776b 634ef32b 00000000
Call Trace:
 [<6001c86d>] show_stack+0xfe/0x15b
 [<6038a835>] dump_stack+0x2a/0x2c
 [<6038776b>] panic+0x13e/0x2b3
 [<6020f099>] btrfs_read_sys_array+0x25d/0x2ff
 [<601cfbbe>] open_ctree+0x192d/0x27af
 [<6019c2c1>] btrfs_mount+0x8f5/0xb9a
 [<600bc9a7>] mount_fs+0x11/0xf3
 [<600d5167>] vfs_kern_mount+0x75/0x11a
 [<6019bcb0>] btrfs_mount+0x2e4/0xb9a
 [<600bc9a7>] mount_fs+0x11/0xf3
 [<600d5167>] vfs_kern_mount+0x75/0x11a
 [<600d710b>] do_mount+0xa35/0xbc9
 [<600d7557>] SyS_mount+0x95/0xc8
 [<6001e884>] handle_syscall+0x6b/0x8e
Reported-by: Jiri Slaby <jslaby@suse.com>
Reported-by: Vegard Nossum <vegard.nossum@oracle.com>
CC: stable@vger.kernel.org	# 3.19+
Signed-off-by: David Sterba <dsterba@suse.com>