JBIG2Stream::readTextRegion: Fix yet another potential integer overflow

oss-fuzz/27783
7 jobs for 27783 in 4 minutes and 49 seconds (queued for 1 second)
Status Job ID Name Coverage
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build

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build_android

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build_clang_libcpp

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build_clazy

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build_mingw64_fedora

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build_ubuntu_16_04

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clang_format

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