patch 8.2.0715: Vim9: leaking memory

Problem:    Vim9: leaking memory.
Solution:   Free strings after concatenating them.

src/version.c

@@ -746,6 +746,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    715,
 /**/
     714,
 /**/

src/vim9compile.c

@@ -4229,6 +4229,8 @@ compile_expr5(char_u **arg, cctx_T *cctx)
 	    }
 	    mch_memmove(tv1.vval.v_string, s1, len1);
 	    STRCPY(tv1.vval.v_string + len1, s2);
+	    vim_free(s1);
+	    vim_free(s2);
 	}
 	else
 	{