fix unwanted integer promotion in tile data size parsing.

Fixes an undefined left shift in
clusterfuzz-testcase-minimized-dav1d_fuzzer-5717082881130496. Credits to
oss-fuzz. Fixes #110
8 jobs for master in 4 minutes and 29 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Build
passed build-debian #5617
debian amd64

00:00:30

passed build-debian-aarch64 #5621
aarch64 debian

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passed build-debian-aarch64-clang-5 #5622
aarch64 clang5 debian

00:01:05

passed build-debian-static #5618
debian amd64

00:00:30

passed build-debian-werror #5624
aarch64 debian

00:00:55

passed build-macos #5623
macos

00:00:23

passed build-win32 #5619
win32

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passed build-win64 #5620
win64

00:00:32