patch 8.2.1860: Vim9: memory leak when throwing empty string

Problem:    Vim9: memory leak when throwing empty string.
Solution:   Free the empty string.

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1860,
 /**/
     1859,
 /**/

src/vim9execute.c

@@ -2094,6 +2094,7 @@ call_def_function(
 		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;
 		}