hdf5_msb

[svn-r21342] Issue 7674 - clang compiler with -fcatch-undefined-behavior -ftrapv discovered several problems in the test suite.  One of
them is in the INIT_INTEGER macro definition in dt_arith.c.  It complained about line 150 where it tried to subtract 1 from
the negative minimal value of "int", causing it to overflow (or underflow).  So I revised the code to avoid it.

Tested on jam, koala, linew, and Mac OS Lion with CLANG compiler.