heap-buffer-overflow in od_block_decode

[tysmith]

Found running commit 4633b81

To reproduce run:
./dump_video test_case.ovg

==47786==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x63400005e800 at pc 0x000000500c31 bp 0x7ffc860e8c10 sp 0x7ffc860e8c08
WRITE of size 16 at 0x63400005e800 thread T0
    #0 0x500c30 in od_block_decode src/decode.c:579:25
    #1 0x4faf86 in od_decode_recursive src/decode.c:782:5
    #2 0x4f61ad in od_decode_coefficients src/decode.c:979:9
    #3 0x4f61ad in daala_decode_packet_in src/decode.c:1204
    #4 0x4ec2de in main examples/dump_video.c:426:13
    #5 0x7f04b6a4f82f in __libc_start_main /build/glibc-GKVZIf/glibc-2.23/csu/../csu/libc-start.c:291
    #6 0x418f98 in _start (dump_video+0x418f98)

test_case.ovg.zip
log.txt