Series sales form: don't allow to submit alternative price without currency

[php-coder]

Regression after #675

[php-coder]

We don't have any affected records

mysql> select count(*) from series_sales where second_price is not null and second_currency is null;
+----------+
| count(*) |
+----------+
|        0 |
+----------+
1 row in set (0.00 sec)

so I'm going to just fix this bug without migrating existing data.

[php-coder]

There is similar bug when it's possible to specify alternative currency only. It won't be shown to user but will be written to database. There are no affected records, and I'm going to fix both bugs as part of this issue.

[0pdd]

@php-coder the puzzle #1011 is still not solved.