Fix subexpression widening in memhash() of disclaim_weakmap_test

Fix subexpression widening in memhash() of disclaim_weakmap_test
(fix of commit 0cc2c0e7e)

It would be more correct to widen the argument (from unsigned to
GC_word) of multiply operation instead of implicit widening of the
result.

* tests/disclaim_weakmap_test.c (memhash): Cast acc to GC_word before
multiplying by 2003; cast the whole expression to unsigned type (before
assigning it to acc).