(inhibit_eval_during_redisplay)

(inhibit_eval_during_redisplay)
(Qinhibit_eval_during_redisplay): New variables.
(safe_eval, safe_call): If inhibit_eval_during_redisplay is set,
don't eval, return nil instead.
(syms_of_xdisp): DEFVAR_BOOL inhibit-eval-during-redisplay.
Initialize Qinhibit_eval_during_redisplay.