cache: fix aborts if no clustering range is specified

cache_streamed_mutation assumed that at least one clustering range was
specified. That was wrong since the readers are allowed to query just
for a static row (e.g. counter update that modifies only static
columns).

Fixes #2604.
Message-Id: <20170725131220.17467-1-pdziepak@scylladb.com>

cache_streamed_mutation.hh

@@ -178,6 +178,10 @@ future<> cache_streamed_mutation::fill_buffer() {
     if (!_static_row_done) {
         _static_row_done = true;
         return process_static_row().then([this] {
+            if (_ck_ranges_curr == _ck_ranges_end) {
+                _end_of_stream = true;
+                return make_ready_future<>();
+            }
             return _lsa_manager.run_in_read_section([this] {
                 return move_to_current_range();
             }).then([this] {