GC problem (0936)

garbage_collect is now called in the should_give tests after the check has
succeeded. Here is one case which caused problem:


[eclipse 1]: lib(test_util).
test_util.pl compiled traceable 7576 bytes in 0.02 seconds

Yes (0.02s cpu)
[eclipse 2]: writeclause(1.0__2.0) should_give true.
1.0__2.0.
GC internal error: bad pointer tag (5) in mark_from

Yes (0.00s cpu)