Fix Crop tool Aspect ratio sometimes randomly preselected

Summary:
If Crop → Advanced settings → Aspect ratio is not set to any of
the predefined values the CropRatioIndex config value is not
initialized when starting Crop. This sometimes leads to a
wrong preselection of a random ratio.
This patch initializes the internal index variable.

FIXED-IN: 18.08.1

Test Plan:

• Open Crop with empty Aspect ratio setting
• Restart Gwenview / Crop multiple times
• Check that Aspect ratio is not randomly preselected
• Check that a selected Aspect ratio is (re-)stored correctly
• Check that gwenviewrc does not store any random CropRatioIndex

Reviewers: Gwenview, rkflx

Reviewed By: Gwenview, rkflx

Subscribers: rkflx

Tags: Gwenview

Differential Revision: https://phabricator.kde.org/D14969