patch 8.2.1860: Vim9: memory leak when throwing empty string

Problem:    Vim9: memory leak when throwing empty string.
Solution:   Free the empty string.
diff --git a/src/vim9execute.c b/src/vim9execute.c
index 31b67d1..673bf91 100644
--- a/src/vim9execute.c
+++ b/src/vim9execute.c
@@ -2094,6 +2094,7 @@
 		if (tv->vval.v_string == NULL
 				       || *skipwhite(tv->vval.v_string) == NUL)
 		{
+		    vim_free(tv->vval.v_string);
 		    emsg(_(e_throw_with_empty_string));
 		    goto failed;
 		}