mutation_result_merger::get() assumes that the merged result may be a
short read if at least one of the partial results is a short read (in
other words, if none of the partial results is a short read, then the
merged result is also not a short read). However this is not true;
because we update the memory accounter incrementally, we may stop
scanning early. All the partial results are full; but we did not scan
the entire range.
Fix by changing the short_read variable initialization from `no`
(which assumes we'll encounter a short read indication when processing
one of the batches) to `this->short_read()`, which also takes into
account the memory accounter.
Fixes#2001.
Message-Id: <20170108111315.17877-1-avi@scylladb.com>