git-gui: Ensure all spellchecker 'class' variables are initialized

git-gui: Ensure all spellchecker 'class' variables are initialized

If we somehow managed to get our spellchecker instance created but
aspell wasn't startable we may not finish _connect and thus may
find one or more of our fields was not initialized in the instance.

If we have an instance but no version, there is no reason to show
a version to the user in our about dialog.  We effectively have no
spellchecker available.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>