msac: Add a cast to indicate intended narrowing from size_t to unsigned

This fixes this compiler warning with MSVC:
../src/msac.c(148): warning C4267: '+=': conversion from 'size_t' to 'unsigned int', possible loss of data
21 jobs for master in 7 minutes and 37 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Style
passed #271932
amd64 debian
style-check

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  Build
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amd64 debian
build-debian

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debian aarch64
build-debian-aarch64

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debian aarch64
build-debian-aarch64-clang-5

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debian armv7
build-debian-armv7

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debian armv7
build-debian-armv7-clang-5

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amd64 debian
build-debian-static

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debian aarch64
build-debian-werror

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amd64 debian
build-debian32

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macos
build-macos

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amd64 debian allowed to fail
build-ubuntu-snap

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passed #271939
amd64 debian
build-win-arm32

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amd64 debian
build-win-arm64

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amd64 debian
build-win32

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amd64 debian
build-win32-unaligned-stack

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amd64 debian
build-win64

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  Test
passed #271948
amd64 debian
test-debian

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amd64 debian
test-debian-asan

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amd64 debian
test-debian-msan

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amd64 debian
test-debian-ubsan

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amd64 debian
test-win64

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