IMPALA-5031: signed overflow is undefined behavior
IMPALA-5031: signed overflow is undefined behavior

This patch fixes a signed overflow in the backend test
DecimalTest.Overflow. The interesting part of the backtrace is:

runtime/decimal-value.inline.h:254:17: runtime error: signed integer
         overflow: 0x4b3b4ca85a86c47a098a223fffffffff +
         0x4b3b4ca85a86c47a098a223fffffffff cannot be represented in
         type '__int128'
    #0 detail::AddLarge(__int128, int, __int128, int, int, bool,
         bool*) runtime/decimal-value.inline.h:254:17
    #1 DecimalValue<__int128> DecimalValue<__int128>::Add<__int128>(
         int, DecimalValue<__int128> const&, int, int, int, bool,
         bool*) const runtime/decimal-value.inline.h:371:14
    #2 DecimalTest_Overflow_Test::TestBody()
         runtime/decimal-test.cc:540:9

Change-Id: I146bcf35d34cc0e14be0633427d3e4bd0e5a261e
Reviewed-on: http://gerrit.cloudera.org:8080/11917
Reviewed-by: Jim Apple <396f577ccd0f19e47f0fb44dae1d6fc43cc20400@apache.org>
Tested-by: Impala Public Jenkins <039b6b20426ff68ebe492b3021890d8c8fd010c9@cloudera.com>

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