BlockRandomAccessSparseMatrix::IntPairToLong suffers from integer
overflow.

Even though the return value of this function is a long int, the
computation happens with three ints, which causes an overflow before
the upgrade happens.

The fix is to upgrade the constant used int his computation to be a
long int, which causes the computation to be done in longs instead of
ints.

A test has been added to verify that the fix works.

Change-Id: Ibb0aef877125bb37ca28754cb07b8e1627fd1d5a