Assert there are some sources when running skpbench

Change-Id: I6009dfc7c6eabe04b3b6fdf68f38995d718b4fef
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/235278
Commit-Queue: Nathaniel Nifong <nifong@google.com>
Reviewed-by: Ben Wagner aka dogben <benjaminwagner@google.com>
3 files changed