An integer overflow in an expression like a+b or a-b is undefined behavior.
The compiler is allowed to assume that no such overflow is possible,
and optimize away some code accordingly.

Item_func_plus::int_op(), Item_func_minus::int_op(): Always check
for overflow.

Depending on the compiler and the compilation options, a test might fail:

CURRENT_TEST: main.func_math
mysqltest: At line 425: query 'SELECT 9223372036854775807 + 9223372036854775807' succeeded - should have failed with errno 1690...

A similar bug had been fixed earlier in
commit 328edf85.