Fixes to String8 fuzzer

Adds support for format fuzzing, fixes several bugs.
Fix: 163680603
Fix: 162926178
Fix: 157469647
Test: libutils_fuzz_string8 clusterfuzz-testcase-minimized-libutils_fuzz_string8-6188353572306944
Test: libutils_fuzz_string8 clusterfuzz-testcase-minimized-libutils_fuzz_string8-6413754773012480
Test: libutils_fuzz_string8 clusterfuzz-testcase-minimized-libutils_fuzz_string8-5691524777246720

Signed-off-by: Dylan Katz <dylan.katz@leviathansecurity.com>
Change-Id: I7107c3637a4befccab345e17171a1119f8677557