don't have fm_bot send fm empty sources to run

Running fm_bot at head I'm confronted with a puzzling error:

    2019/04/03 10:26:01
    out/fm -b cpu -w foo -s big_rrect_rect_aa_effect filterbitmap_image_mandrill_512.png                    #failed (exit status 1):
    Don't understand --source ... bailing out.

    2019/04/03 10:26:05 1 invocations of out/fm failed

And of course running that command works fine.  Puzzling!

To fix this, we must take care not to subslice sources past its end,
which fills with defaults, a bunch of extra "" sources.  And add some ''
around the unknown source to make the error message easier to parse.

Serves me right for not testing this before I landed that tweak...

Change-Id: I5c2abb35733b32a1142b0f42c4323112969ad2bb
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/205825
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
Auto-Submit: Mike Klein <mtklein@google.com>
2 files changed