Fix Segfault for LONGTEXT in bind_result

It actually does not fix the bug itself because PHP behaves the same.
The cause of the segfault is that some people run out of memory resulting in a bad call to calloc.
calloc returns a nullptr which later in mysql is dereferenced.

PHP's solution to this is to call store_result before binding. But this will also fail in hhvm.
This fixes it.

TL;DR: TEXT in mysql is now variable length instead of 4GB when calling store_result

Fixes #2377
@SiebelsTim Hi. I got some feedback internally from @WizKid...

> That looks wrong to me. It is in the wrong place at least. The idea with MySQLStmt methods was that they were going to match one to one with the C-api. There already exists a `MySQLStmt::attr_set` method. So if we need to call that before we should do that. We shouldn't do stuff in `::store_result`.

> We also have a bunch of logic around `STMT_ATTR_UPDATE_MAX_LENGTH` and php-src has it to. So just adding it here, I'm pretty sure is the wrong thing.