Update mysqlc's ColumnLocate::findColumn to throw for invalid column.

(This is to comply with the updated API specification.)

Change-Id: I4542fecc78a6e64011276dafc72c31d5533af1ab
Reviewed-on: https://gerrit.libreoffice.org/5923Reviewed-by: Fridrich Strba <fridrich@documentfoundation.org>
Tested-by: Fridrich Strba <fridrich@documentfoundation.org>

mysqlc/source/mysqlc_resultset.cxx

@@ -179,7 +179,13 @@ sal_Int32 SAL_CALL OResultSet::findColumn(const OUString& columnName)
     } catch (const sql::SQLException &e) {
         mysqlc_sdbc_driver::translateAndThrow(e, *this, m_encoding);
     }
-    return 0;
+    throw SQLException(
+        "The column name '" + columnName + "' is not valid.",
+        *this,
+        OUString("42S22"),
+        0,
+        Any()
+    );
 }
 /* }}} */