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/screen.c b/src/screen.c
index ea7aaa6..01d6257 100644
--- a/src/screen.c
+++ b/src/screen.c
@@ -1148,7 +1148,7 @@
 	curwin = wp;
 	STRCPY(buf, "b:keymap_name");	// must be writable
 	++emsg_skip;
-	s = p = eval_to_string(buf, NULL, FALSE);
+	s = p = eval_to_string(buf, FALSE);
 	--emsg_skip;
 	curbuf = old_curbuf;
 	curwin = old_curwin;