patch 8.2.1049: Vim9: leaking memory when using continuation line

Problem:    Vim9: leaking memory when using continuation line.
Solution:   Keep a pointer to the continuation line in evalarg_T.  Centralize
            checking for a next command.
diff --git a/src/buffer.c b/src/buffer.c
index f928412..40ca25d 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -4094,7 +4094,7 @@
 	tv.vval.v_number = wp->w_id;
 	set_var((char_u *)"g:statusline_winid", &tv, FALSE);
 
-	usefmt = eval_to_string_safe(fmt + 2, NULL, use_sandbox);
+	usefmt = eval_to_string_safe(fmt + 2, use_sandbox);
 	if (usefmt == NULL)
 	    usefmt = fmt;
 
@@ -4434,7 +4434,7 @@
 	    if (curwin != save_curwin)
 		VIsual_active = FALSE;
 
-	    str = eval_to_string_safe(p, &t, use_sandbox);
+	    str = eval_to_string_safe(p, use_sandbox);
 
 	    curwin = save_curwin;
 	    curbuf = save_curbuf;