`this._markers` is undefined in `_searchPoints`, called from event listeners.
Now we check markers existence in the beginning of handlers.

Note 1:
Could be fixed in some another place:
 e.g. check `_markers` in `_searchPoints`
 ...or initialize (empty) `_markers` on CanvasMarkerLayer init

Note 2:
`!this._map` removed from condition in `_onMouseMove:` 'cause
it's never should be true as we detach listeners on remove from map

… map

Was protected in marker add time, but not on zoom/pan

Sample:
```
VM2992:16252 Uncaught DOMException: Failed to execute 'drawImage' on 'CanvasRenderingContext2D': The HTMLImageElement provided is in the 'broken' state.
    at NewClass._drawImage (<anonymous>:16252:23)
    at NewClass._drawMarker (<anonymous>:16239:22)
    at <anonymous>:16200:22
    at Array.forEach (<anonymous>)
    at NewClass._draw (<anonymous>:16179:16)
    at NewClass._redraw (<anonymous>:16077:18)
    at NewClass.fire (<anonymous>:1867:11)
    at NewClass._move (<anonymous>:5506:9)
    at NewClass._onZoomTransitionEnd (<anonymous>:5963:8)
```