Rewrite setIndex(int) to pass unit tests

Original did not cover the case of when streams is empty and
documentation does not specify any input restrictions.

There's an ambiguity with broadcasting an event between the
documentation and the actual code (see TODO).

Author: imericxu

app/src/main/java/org/schabi/newpipe/player/playqueue/PlayQueue.java

@@ -135,18 +135,36 @@ public int getIndex() {
     public synchronized void setIndex(final int index) {
         final int oldIndex = getIndex();
 
-        int newIndex = index;
+        final int newIndex;
+
         if (index < 0) {
             newIndex = 0;
+        } else if (index < streams.size()) {
+            // Regular assignment for index in bounds
+            newIndex = index;
+        } else if (streams.isEmpty()) {
+            // Out of bounds from here on
+            // Need to check if stream is empty to prevent arithmetic error and negative index
+            newIndex = 0;
+        } else if (isComplete()) {
+            // Circular indexing
+            newIndex = index % streams.size();
+        } else {
+            // Index of last element
+            newIndex = streams.size() - 1;
         }
-        if (index >= streams.size()) {
-            newIndex = isComplete() ? index % streams.size() : streams.size() - 1;
-        }
+
+        queueIndex.set(newIndex);
+
         if (oldIndex != newIndex) {
             history.add(streams.get(newIndex));
         }
 
-        queueIndex.set(newIndex);
+        /*
+        TODO: Documentation states that a SelectEvent will only be emitted if the new index is...
+        different from the old one but this is emitted regardless? Not sure what this what it does
+        exactly so I won't touch it
+         */
         broadcast(new SelectEvent(oldIndex, newIndex));
     }