shift exponent -1 is negative in tile_log2() src/obu.c

Found with commit acd90b71

Steps to reproduce:

build dav1d with UBSan (-fsanitize=shift)
run attached testcase with dav1d executable ./dav1d -i testcase.ivf -o out.ivf

src/env.h:296:24: runtime error: shift exponent -1 is negative
    #0 0x502ddc in tile_log2 src/obu.c:311:21
    #1 0x502ddc in parse_frame_hdr src/obu.c:435
    #2 0x4fa78d in dav1d_parse_obus src/obu.c:1023:20
    #3 0x4f6687 in dav1d_decode src/lib.c:193:20
    #4 0x4eaa77 in main tools/dav1d.c:108:20
    #5 0x7fbf448a882f in __libc_start_main /build/glibc-Cl5G7W/glibc-2.23/csu/libc-start.c:291
    #6 0x418d38 in _start (dav1d+0x418d38)

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