Fix a crash which occurs when RTL flag is turned on.

Fix a crash which occurs when RTL flag is turned on.

The crash was being caused by the fact that when moving back from the
beginning of the line to the previous line (by pressing RIGHT in an
RTL paragraph, or LEFT in an LTR one), Bidi metrics for the new line
do not yet exist, but an attempt is made to access them.

Basically, this patch was created by trying to "symmetrically" copy
cursorRight (which seems to work well) to cursorLeft (which was
problematic).

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@18389 a592a061-630c-0410-9148-cb99ea01b6c8