Fix userspace fastboot with fuzzy test

Add more checking for fastboot to detect malformed
requests.
Such as checking no control characters in the command
send from host.
Make sure the download command length is eight bytes.
And report FAIL if download length is zero.

Test: adb reboot fastboot
      fuzzy_fastboot --gtest_filter=Fuzz.DownloadInvalid1
      fuzzy_fastboot --gtest_filter=Fuzz.DownloadInvalid2
      fuzzy_fastboot --gtest_filter=Fuzz.DownloadInvalid7
      fuzzy_fastboot --gtest_filter=Fuzz.DownloadInvalid8
Bug: 212628476
Change-Id: I750174205377395b5328923fb00462d078f3310d