Avoid segfaults in j9vmem_testOverlappingSegments

[cjjdespres]

Since the keepCycles array is zero-initialized, if the test is aborted in the very first cycle (i == 0) the cleanup procedure at the end cannot rely on the fact that keepCycles[j] >= i implies that segment j must be freed. Before these changes, such a scenario would cause the test to segfault during cleanup.

Fixes: https://github.com/eclipse-openj9/openj9/issues/19509