Fix symbol relocation when the relocated cell is renamed.

Fix symbol relocation when the relocated cell is renamed.

* lisp/ses.el (ses-sym-rowcol):  Check that the renamed cell
hashmap has been instantiated before getting data from it.  When
editing several spreadsheets, and you have spreadsheet #1 with a
cell named `foo', and no renamed cell in spreadsheet #2, then if
you make a formula with `foo' in spreadsheet #2, not doing this
check will make an error.
(ses-cell-set-formula): Robustify versus incorrect cell references
given in the user provided formula.  An explicit error message is
provided after the action when the user gives an incorrect cell
reference, but the formula edition is not changed.  This means that
if the incorrect reference is to a cell that is created someday,
then this new cell will not have the edited cell in its reference
list.  Fixing this can still be done by editing again the first
cell formula.
(ses-relocate-symbol): Do not create symbol of referred-to cell
when this is a renamed cell.