Make sure fuzzer can't pick an illegal colortype

BUG=skia:6216

Change-Id: Ifb0a0a1e634bb291c586d2094401ec10349dcd0e
Reviewed-on: https://skia-review.googlesource.com/8817
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Kevin Lubick <kjlubick@google.com>
1 file changed