patch 8.1.1591: on error garbage collection may free memory in use

Problem:    On error garbage collection may free memory in use.
Solution:   Reset may_garbage_collect when evaluating expression mapping.
            Add tests. (Ozaki Kiichi, closes #4579)
diff --git a/src/ex_cmds2.c b/src/ex_cmds2.c
index 8871f5d..036a2a7 100644
--- a/src/ex_cmds2.c
+++ b/src/ex_cmds2.c
@@ -367,8 +367,8 @@
 	    int save_vgetc_busy = vgetc_busy;
 	    int save_did_emsg = did_emsg;
 	    int save_called_emsg = called_emsg;
-	    int	save_must_redraw = must_redraw;
-	    int	save_trylevel = trylevel;
+	    int save_must_redraw = must_redraw;
+	    int save_trylevel = trylevel;
 	    int save_did_throw = did_throw;
 	    int save_ex_pressedreturn = get_pressedreturn();
 	    int save_may_garbage_collect = may_garbage_collect;