Fixes #152, #153. Fixes a global buffer overflow in obmc() with
clusterfuzz-testcase-dav1d_fuzzer_mt-5702455078158336 and an ubsan
Index-out-of-bounds error in dav1d_recon_b_inter_8bpc() with
clusterfuzz-testcase-minimized-dav1d_fuzzer_mt-5688109887389696. Credits
to oss-fuzz.