intrabc: use visible width/height in mv correction

Prevents adjusting intra block copy motion vectors to values pointing
out of the current tile. This happens with not entirely visible blocks in
a one super block wide/high tile. Fixes an use of uninitilized value in
inv_txfm_add_c() with
clusterfuzz-testcase-minimized-dav1d_fuzzer_mt-5746740678885376. Credits
to oss-fuzz.
12 jobs for master in 4 minutes and 50 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
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debian aarch64
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  Test
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amd64 debian
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