ICU-20824 Fix - avoid potential out-of-bounds array access (undefined behaviour)

[oshai]

This is a backport of a fix from mapnik clone of this class: mapnik/mapnik#4096

Also suggested to icu as:
https://unicode-org.atlassian.net/browse/ICU-20824

We encountered it in mapnik and fixed it in our own mapnik 3.0 fork. I think it worth backporting the fix to here as well.

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[markusicu]

Could you please add a test case? Presumably in srtest.cpp.

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[oshai]

Could you please add a test case? Presumably in srtest.cpp.

added the test case from the jira issue.