oss-fuzz: MemorySanitizer: use-of-uninitialized-value in selfguided_filter
reproduce with ./tests/dav1d_fuzzer clusterfuzz-testcase-minimized-dav1d_fuzzer-5710557533962240
clusterfuzz-testcase-minimized-dav1d_fuzzer-5710557533962240
==919==WARNING: MemorySanitizer: use-of-uninitialized-value
#0 0x7fa4100536c7 in selfguided_filter (/home/janne/src/dav1d/build-msan-fuzz/tests/../src/libdav1d.so.0+0x1b36c7)
#1 0x7fa41004edd2 in selfguided_c (/home/janne/src/dav1d/build-msan-fuzz/tests/../src/libdav1d.so.0+0x1aedd2)
#2 0x7fa41004cf73 in lr_sbrow (/home/janne/src/dav1d/build-msan-fuzz/tests/../src/libdav1d.so.0+0x1acf73)
#3 0x7fa41004b670 in dav1d_lr_sbrow_16bpc (/home/janne/src/dav1d/build-msan-fuzz/tests/../src/libdav1d.so.0+0x1ab670)
#4 0x7fa41006f48b in dav1d_filter_sbrow_16bpc (/home/janne/src/dav1d/build-msan-fuzz/tests/../src/libdav1d.so.0+0x1cf48b)
#5 0x7fa40ff1c3db in dav1d_decode_frame (/home/janne/src/dav1d/build-msan-fuzz/tests/../src/libdav1d.so.0+0x7c3db)
#6 0x7fa40ff22007 in dav1d_submit_frame (/home/janne/src/dav1d/build-msan-fuzz/tests/../src/libdav1d.so.0+0x82007)
#7 0x7fa40feff422 in dav1d_parse_obus (/home/janne/src/dav1d/build-msan-fuzz/tests/../src/libdav1d.so.0+0x5f422)
#8 0x7fa410073abe in dav1d_decode (/home/janne/src/dav1d/build-msan-fuzz/tests/../src/libdav1d.so.0+0x1d3abe)
#9 0x4c67df in LLVMFuzzerTestOneInput (/home/janne/src/dav1d/build-msan-fuzz/tests/dav1d_fuzzer+0x4c67df)
#10 0x42e062 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) /var/tmp/portage/sys-libs/compiler-rt-sanitizers-7.0.0/work/compiler-rt-7.0.0.src/lib/fuzzer/FuzzerLoop.cpp:576:15
#11 0x41fbad in fuzzer::RunOneTest(fuzzer::Fuzzer*, char const*, unsigned long) /var/tmp/portage/sys-libs/compiler-rt-sanitizers-7.0.0/work/compiler-rt-7.0.0.src/lib/fuzzer/FuzzerDriver.cpp:280:6
#12 0x424782 in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) /var/tmp/portage/sys-libs/compiler-rt-sanitizers-7.0.0/work/compiler-rt-7.0.0.src/lib/fuzzer/FuzzerDriver.cpp:714:9
#13 0x44d642 in main /var/tmp/portage/sys-libs/compiler-rt-sanitizers-7.0.0/work/compiler-rt-7.0.0.src/lib/fuzzer/FuzzerMain.cpp:20:10
#14 0x7fa40ef2cae6 in __libc_start_main (/lib64/libc.so.6+0x21ae6)
#15 0x41df69 in _start (/home/janne/src/dav1d/build-msan-fuzz/tests/dav1d_fuzzer+0x41df69)