bug-fix -- adding negative num to zero used to give a positive result.

[Linas Vepstas]

Log Message:
-----------
bug-fix -- adding negative num to zero used to give a positive result.
(this bug was introduced only recently, about a week ago, with the changes
to 128-bit intermediate calculations, and does not/should not affect any
actual users).

Modified Files:
--------------
    gnucash/src/engine:
        gnc-numeric.c