Clean up screenshot if nothing selected

If rect is null, run the finisher so the screenshot logic
is cleaned up and prepared for next screenshot. This fixes
BUGBASH-1831.

[basilgello: combined two if-clauses to avoid double else,
  inspired by @BadDaemon]
Signed-off-by: Vasyl Gello <vasek.gello@gmail.com>

Change-Id: Ib46221df2a2a70bc5a901107ca6aa4268a0568f2