Add checking for sparse file format

Sparse file can come from an untrusted source.
Need more checking to ensure that it is not a malformed
file and would not cause any OOB read access.

Update fuzz test for decoding also.

Test: adb reboot fastboot
      fuzzy_fastboot --gtest_filter=Fuzz.Sparse*
      fuzzy_fastboot --gtest_filter=Conformance.Sparse*
      sparse_fuzzer
Bug: 212705418
Change-Id: I7622df307bb00e59faaba8bb2c67cb474cffed8e