Fix for PR 17148

[Marshall Clow]

http://llvm.org/bugs/show_bug.cgi?id=17148

Fix an edge case where the size would overflow, giving a much smaller buffer than expected.
Add a test, too.

-- Marshall

Marshall Clow     Idio Software   <mailto:mclow.lists at gmail.com>

A.D. 1517: Martin Luther nails his 95 Theses to the church door and is promptly moderated down to (-1, Flamebait).
        -- Yu Suzuki
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