mysql_stmt_fetch_column() should work without mysql_stmt_store_result()

It seems that fetching columns of blob or varchar type via mysql_stmt_fetch_column() requires a call to mysql_stmt_store_result() after mysql_stmt_execute(). Otherwise all attempts to fetch the data result in CR_NO_DATA. This is different in the MySQL C connector, and I don't see why I need to bring the entire result set to the client just to make available all blobs in a row. (At least this is how I understand the documentation.) This might severely impact the performance of the new RMariaDB database backend for R.

Removing all calls to mysql_stmt_store_result() from the tests gives two failures, see [1] for a patch:

1. port: 0
2. socketname: (null)
   1..13
3. Testing against MySQL Server 5.7.19-0ubuntu0.17.04.1
4. Host: Localhost via UNIX socket
5. Client library: 10.2.6
6. Error: (/home/muelleki/git/mariadb-connector-c/unittest/libmariadb/fetch.c: 241)
   not ok 1 - test_fetch_seek
7. truncation: 1
8. Error: (/home/muelleki/git/mariadb-connector-c/unittest/libmariadb/fetch.c: 316)
   not ok 2 - test_fetch_offset
   ok 3 - test_fetch_column
   ok 4 - test_fetch_nobuffs
   ok 5 - test_fetch_null
   ok 6 - test_fetch_date
   ok 7 - test_fetch_str
   ok 8 - test_fetch_long
   ok 9 - test_fetch_short
   ok 10 - test_fetch_tiny
   ok 11 - test_fetch_bigint
   ok 12 - test_fetch_float
   ok 13 - test_fetch_double
9. close default
10. Failed 2 tests!

[1] https://github.com/krlmlr/mariadb-connector-c/commit/5db25b0e0448046adec7bdf8ee8cbf47228be5ac