Add libfstab_fuzzer that fuzzes ReadFstabFile()

Just a modest initial implementation. Uses the fuzz data as the fstab
file content directly.

Bug: 204056804
Test: lunch aosp_cf_x86_64_phone-userdebug
  SANITIZE_TARGET=address m libfstab_fuzzer
  adb sync data
  adb shell /data/fuzz/x86_64/libfstab_fuzzer/libfstab_fuzzer
Change-Id: I7976a6ee124e9b5da59cfa7f4bae9699be3f1474