Remove `std::abort` usage

[CuriousGeorgiy]

We use std::abort in two places:

tntcxx/src/Client/Connection.hpp

Lines 404 to 414 in b89ebc6

	template<class BUFFER, class NetProvider>
	Response<BUFFER>
	Connection<BUFFER, NetProvider>::getResponse(rid_t future)
	{
	//This method does not tolerate extracting wrong future.
	//Firstly user should invoke futureIsReady() to make sure future
	//is present.
	auto entry = impl->futures.find(future);
	#ifndef NDEBUG
	if (entry == impl->futures.end())
	std::abort();

tntcxx/src/Client/Connection.hpp

Lines 531 to 549 in b89ebc6

	template<class BUFFER, class NetProvider>
	DecodeStatus
	processResponse(Connection<BUFFER, NetProvider> &conn,
	Response<BUFFER> *result)
	{
	//Decode response. In case of success - fill in feature map
	//and adjust end-of-decoded data pointer. Call GC if needed.
	if (! conn.impl->inBuf.has(conn.impl->endDecoded, MP_RESPONSE_SIZE))
	return DECODE_NEEDMORE;
	Response<BUFFER> response;
	response.size = conn.impl->dec.decodeResponseSize();
	if (response.size < 0) {
	LOG_ERROR("Failed to decode response size");
	//In case of corrupted response size all other data in the buffer
	//is likely to be decoded in the wrong way (since we don't
	// know how much bytes should be skipped). So let's simply
	//terminate here.
	std::abort();

This is unacceptable for library code — we need to inspect these places and find an alternative.