Fix stalling when using buffer.fastSwitchEnabled setting (#4004)

According to the docs in Settings, the value used by StreamProcessor was wrong.
The problem occurred on an environment with high latency and low bandwidth, where segments' length was 3s, which is significantly longer than previously hardcoded 1.5s.

src/streaming/StreamProcessor.js

@@ -660,7 +660,7 @@ function StreamProcessor(config) {
     function _prepareForFastQualitySwitch(representationInfo) {
         // if we switch up in quality and need to replace existing parts in the buffer we need to adjust the buffer target
         const time = playbackController.getTime();
-        let safeBufferLevel = 1.5;
+        let safeBufferLevel = 1.5 * (!isNaN(representationInfo.fragmentDuration) ? representationInfo.fragmentDuration : 1);
         const request = fragmentModel.getRequests({
             state: FragmentModel.FRAGMENT_MODEL_EXECUTED,
             time: time + safeBufferLevel,