use-of-uninitialized-value in dav1d_loopfilter_sbrow_8bpc() src/mc.c

Found with commit acd90b71

Steps to reproduce:

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

testcase.ivf

==7977==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x723782 in dav1d_loopfilter_sbrow_8bpc src/lf_apply.c:185:85
    #1 0x64598c in dav1d_filter_sbrow_8bpc src/recon.c:1456:9
    #2 0x4d47d4 in dav1d_decode_frame src/decode.c:2576:25
    #3 0x4df3c1 in dav1d_submit_frame src/decode.c:2956:20
    #4 0x4ba9bb in dav1d_parse_obus src/obu.c:1075:20
    #5 0x49e2ee in dav1d_decode src/lib.c:193:20
    #6 0x487b24 in main tools/dav1d.c:108:20
    #7 0x7f7459af482f in __libc_start_main /build/glibc-Cl5G7W/glibc-2.23/csu/libc-start.c:291
    #8 0x419b48 in _start (dav1d+0x419b48)

  Uninitialized value was created by a heap allocation
    #0 0x420a10 in __interceptor_malloc (dav1d+0x420a10)
    #1 0x4ced48 in dav1d_decode_frame src/decode.c:2419:38
    #2 0x4df3c1 in dav1d_submit_frame src/decode.c:2956:20
    #3 0x4ba9bb in dav1d_parse_obus src/obu.c:1075:20
    #4 0x49e2ee in dav1d_decode src/lib.c:193:20
    #5 0x487b24 in main tools/dav1d.c:108:20
    #6 0x7f7459af482f in __libc_start_main /build/glibc-Cl5G7W/glibc-2.23/csu/libc-start.c:291