Resolved UAF issue in RefBase fuzzer

Restrucures this fuzzer to be far more robust and less brittle.
Fix: 163727995
Fix: 163449137
Test: libutils_fuzz_refbase clusterfuzz-testcase-minimized-libutils_fuzz_refbase-5674315436261376
Test: libutils_fuzz_refbase clusterfuzz-testcase-minimized-libutils_fuzz_refbase-5731662044069888
Test: libutils_fuzz_refbase clusterfuzz-testcase-minimized-libutils_fuzz_refbase-5081777218256896

Signed-off-by: Dylan Katz <dylan.katz@leviathansecurity.com>
Change-Id: I239298dc2895a06af5a126e9ca2ae452579e5cc0