Fix fuzzer so it doesn't add overlapping maps.

The fuzzer was creating individual maps that overlapped with other maps.
Since this is not possible in the real world unless the kernel is broken,
do not let the fuzzer do this. This resulted in memory leaks, because some
parts of the code have this assumption baked in.

Bug: 160895854

Test: Ran fuzzer test case that leaked memory and verified it no longer does.
Change-Id: I9f3c1e28781093b041b747e1566fb51d40d2bf71