Bug#21042 mysql client segfaults on importing a mysqldump export

- Use strmake, that will both protect the buffer and make sure it's terminated by a zero
 - Add test case


client/mysql.cc:
  Use strmake instead of strxnmov as that function will make sure the string is terminated by 0
mysql-test/r/mysql.result:
  Update test result
mysql-test/t/mysql.test:
  Add test case

client/mysql.cc

@@ -2858,7 +2858,7 @@ com_connect(String *buffer, char *line)
   bzero(buff, sizeof(buff));
   if (buffer)
   {
-    strxnmov(buff, sizeof(buff), line, NullS);
+    strmake(buff, line, sizeof(buff));
     tmp= get_arg(buff, 0);
     if (tmp && *tmp)
     {