Assertion `ref_frame_type > NONE_FRAME' failed in av1_set_ref_frame() src/ref_mvs.c

Found with commit acd90b71

Steps to reproduce:

1. run attached testcase with dav1d executable ./dav1d -i testcase.ivf -o out.ivf

testcase.ivf

src/ref_mvs.c:1353: void av1_set_ref_frame(MV_REFERENCE_FRAME *, int8_t): Assertion `ref_frame_type > NONE_FRAME' failed.
    #0 0x7f02c7800427 in gsignal /build/glibc-Cl5G7W/glibc-2.23/signal/../sysdeps/unix/sysv/linux/raise.c:54
    #1 0x7f02c7802029 in abort /build/glibc-Cl5G7W/glibc-2.23/stdlib/abort.c:89
    #2 0x7f02c77f8bd6 in __assert_fail_base /build/glibc-Cl5G7W/glibc-2.23/assert/assert.c:92
    #3 0x7f02c77f8c81 in __assert_fail /build/glibc-Cl5G7W/glibc-2.23/assert/assert.c:101
    #4 0x57ad72 in av1_set_ref_frame src/ref_mvs.c:1353:5
    #5 0x57ad72 in av1_find_mv_refs src/ref_mvs.c:2399
    #6 0x582dc0 in av1_find_ref_mvs src/ref_mvs.c:3365:5
    #7 0x52ab29 in decode_b src/decode.c:1249:13
    #8 0x50e3f8 in decode_sb src/decode.c:1986:13
    #9 0x50e12e in decode_sb src/decode.c:1848:21
    #10 0x50c27a in dav1d_decode_tile_sbrow src/decode.c:2228:13
    #11 0x515f72 in dav1d_decode_frame src/decode.c:2571:29
    #12 0x51ccd1 in dav1d_submit_frame src/decode.c:2956:20
    #13 0x504298 in dav1d_parse_obus src/obu.c:1075:20
    #14 0x4f5f87 in dav1d_decode src/lib.c:193:20
    #15 0x4eaa77 in main tools/dav1d.c:108:20
    #16 0x7f02c77eb82f in __libc_start_main /build/glibc-Cl5G7W/glibc-2.23/csu/../csu/libc-start.c:291
    #17 0x418d38 in _start (dav1d+0x418d38)