do not crash in user-defined operations if the controller is disabled

Scylla currently crashes if we run manual operations like nodetool
compact with the controller disabled. While we neither like nor
recommend running with the controller disabled, due to some corner cases
in the controller algorithm we are not yet at the point in which we can
deprecate this and are sometimes forced to disable it.

The reason for the crash is that manual operations will invoke
_backlog_of_shares, which returns what is the backlog needed to
create a certain number of shares. That scan the existing control
points, but when we run without the controller there are no control
points and we crash.

Backlog doesn't matter if the controller is disabled, and the return
value of this function will be immaterial in this case. So to avoid the
crash, we return something right away if the controller is disabled.

Fixes #5016

Signed-off-by: Glauber Costa <glauber@scylladb.com>
(cherry picked from commit c9f2d1d105)

database.cc

@@ -2233,6 +2233,10 @@ void backlog_controller::adjust() {
 
 float backlog_controller::backlog_of_shares(float shares) const {
     size_t idx = 1;
+    // No control points means the controller is disabled.
+    if (_control_points.size() == 0) {
+            return 1.0f;
+    }
     while ((idx < _control_points.size() - 1) && (_control_points[idx].output < shares)) {
         idx++;
     }