Avoid core while extracting error message from future w/NULL response.

[angelcaban]

When attempting to get future error result from future without response, got the following stack trace:

#0  0x00007ffff10d5f7d in boost::atomics::detail::base_atomic<int, int, 4u, true>::fetch_add (this=0x8, v=1, order=boost::memory_order_relaxed)
    at include/boost/atomic/detail/gcc-x86.hpp:612
#1  0x00007ffff10d48d8 in cass::Atomic<int>::fetch_add (this=0x8, value=1, order=cass::MEMORY_ORDER_RELAXED)
    at cpp-driver/src/atomic/atomic_boost.hpp:48
#2  0x00007ffff10dd236 in cass::RefCounted<cass::Response>::inc_ref (this=0x8) at cpp-driver/src/ref_counted.hpp:42
#3  0x00007ffff114fcda in cass_future_get_error_result (future=0x7fff780017c0) at cpp-driver/src/future.cpp:101
#4  0x00007ffff6f7b179 in cassutils::buildErrorFromFuture (pFuture=0x7fff780017c0, errorOut="Invalid query") at 

[datastax-bot]

Hi @angelcaban, thanks for your contribution!

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[datastax-bot]

Thank you @angelcaban for signing the Contribution License Agreement.

Cheers,
DataStax Bot.

[mpenick]

Thanks for the PR. This line could probably be moved into the previous check.

   if (result) {
      result->decode_first_row();
      result->inc_ref();  // Move it here
    }

[mpenick]

What do you think about creating a new error for this? I skimmed over the existing errors codes and couldn't find one that fits this type of error. I'm thinking something like 'CASS_ERROR_LIB_RESPONSE_NOT_AVAILABLE`.