Avoid Dump in Merge Activity Wizard

... Only allow "Next" if 2nd file is loaded (otherwise a dump occurs)

(cherry picked from commit 88b8479cbb90c8306ca32c0d193199ed895db95c)
Merge pull request #893 from Joern-R/pull

Avoid Dump in Merge Activity Wizard
Fixup Fix for MergeActivityWizard crash

.. TRUE/FALSE are case sensitive on Mac
.. ride2Loaded renamed ridesLoaded
.. ridesLoaded initialisation moved to constructor

Thanks to Jorn for the fix to a P0 bug !
Remove unneeded bool in MergeWizard

.. use the pointer to the loaded ride instead
.. but also make sure it gets initialised too !
Merge Upstream Master