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/beval.c b/src/beval.c
index b7d9226..dd7bc3c 100644
--- a/src/beval.c
+++ b/src/beval.c
@@ -285,7 +285,7 @@
 	    ++textwinlock;
 
 	    vim_free(result);
-	    result = eval_to_string(bexpr, NULL, TRUE);
+	    result = eval_to_string(bexpr, TRUE);
 
 	    // Remove one trailing newline, it is added when the result was a
 	    // list and it's hardly ever useful.  If the user really wants a