global-buffer-overflow in od_compute_thresh

[tysmith]

Found running commit 4633b81
Built with AddressSanitizer

To reproduce run:
./dump_video test_case.ogv

==46890==ERROR: AddressSanitizer: global-buffer-overflow on address 0x0000005b3220 at pc 0x00000050cc54 bp 0x7ffdbb0c90d0 sp 0x7ffdbb0c90c8
READ of size 2 at 0x0000005b3220 thread T0
    #0 0x50cc53 in od_compute_thresh src/dering.c:247:35
    #1 0x50cc53 in od_dering src/dering.c:286
    #2 0x4f7bfe in od_decode_coefficients src/decode.c:1069:13
    #3 0x4f7bfe in daala_decode_packet_in src/decode.c:1204
    #4 0x4ec2de in main examples/dump_video.c:426:13
    #5 0x7ff4aadcf82f 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.ogv.zip