cmd: Don't overflow parameter length when copying substring.

programs/cmd/tests/test_builtins.cmd.exp

@@ -275,7 +275,7 @@ P0S
 q
 qwe
 er
-@todo_wine@'ty'@or_broken@''
+'ty'@or_broken@''
 't'@or_broken@''
 ert@or_broken@qwerty
 e@or_broken@qwerty

programs/cmd/wcmdmain.c

@@ -730,6 +730,7 @@ static WCHAR *WCMD_expand_envvar(WCHAR *start,
         else if (copybytes < 0) copybytes = 0;
         WCMD_strsubstW(start, endOfVar + 1, startCopy, copybytes);
       } else {
+        substrlength = min(substrlength, len - (startCopy- thisVarContents + 1));
         WCMD_strsubstW(start, endOfVar + 1, startCopy, substrlength);
       }