Fixed issue when VOD-style period is present in a dynamic manifest

A VOD-style period in a dynamic manifest would result in a NullPointerException.
Also fix another issue in which an unrecognized mime type would also result in
NullPointerException.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=138075137

library/src/main/java/com/google/android/exoplayer2/source/dash/DashMediaSource.java

@@ -424,9 +424,13 @@ public final class DashMediaSource implements MediaSource {
         // not correspond to the start of a segment in both, but this is an edge case.
         DashSegmentIndex index =
             period.adaptationSets.get(videoAdaptationSetIndex).representations.get(0).getIndex();
-        int segmentNum = index.getSegmentNum(defaultStartPositionInPeriodUs, periodDurationUs);
-        windowDefaultStartPositionUs =
-            defaultStartPositionUs - defaultStartPositionInPeriodUs + index.getTimeUs(segmentNum);
+        if (index != null) {
+          int segmentNum = index.getSegmentNum(defaultStartPositionInPeriodUs, periodDurationUs);
+          windowDefaultStartPositionUs =
+              defaultStartPositionUs - defaultStartPositionInPeriodUs + index.getTimeUs(segmentNum);
+        } else {
+          windowDefaultStartPositionUs = defaultStartPositionUs;
+        }
       } else {
         windowDefaultStartPositionUs = defaultStartPositionUs;
       }

library/src/main/java/com/google/android/exoplayer2/text/SubtitleDecoderFactory.java

@@ -81,6 +81,9 @@ public interface SubtitleDecoderFactory {
     }
 
     private Class<?> getDecoderClass(String mimeType) {
+      if (mimeType == null) {
+        return null;
+      }
       try {
         switch (mimeType) {
           case MimeTypes.TEXT_VTT:

library/src/main/java/com/google/android/exoplayer2/util/MimeTypes.java

@@ -84,7 +84,7 @@ public final class MimeTypes {
    * @return Whether the top level type is audio.
    */
   public static boolean isAudio(String mimeType) {
-    return getTopLevelType(mimeType).equals(BASE_TYPE_AUDIO);
+    return BASE_TYPE_AUDIO.equals(getTopLevelType(mimeType));
   }
 
   /**
@@ -94,7 +94,7 @@ public final class MimeTypes {
    * @return Whether the top level type is video.
    */
   public static boolean isVideo(String mimeType) {
-    return getTopLevelType(mimeType).equals(BASE_TYPE_VIDEO);
+    return BASE_TYPE_VIDEO.equals(getTopLevelType(mimeType));
   }
 
   /**
@@ -104,7 +104,7 @@ public final class MimeTypes {
    * @return Whether the top level type is text.
    */
   public static boolean isText(String mimeType) {
-    return getTopLevelType(mimeType).equals(BASE_TYPE_TEXT);
+    return BASE_TYPE_TEXT.equals(getTopLevelType(mimeType));
   }
 
   /**
@@ -114,7 +114,7 @@ public final class MimeTypes {
    * @return Whether the top level type is application.
    */
   public static boolean isApplication(String mimeType) {
-    return getTopLevelType(mimeType).equals(BASE_TYPE_APPLICATION);
+    return BASE_TYPE_APPLICATION.equals(getTopLevelType(mimeType));
   }
 
 
@@ -237,9 +237,12 @@ public final class MimeTypes {
    * Returns the top-level type of {@code mimeType}.
    *
    * @param mimeType The mimeType whose top-level type is required.
-   * @return The top-level type.
+   * @return The top-level type, or null if the mimeType is null.
    */
   private static String getTopLevelType(String mimeType) {
+    if (mimeType == null) {
+      return null;
+    }
     int indexOfSlash = mimeType.indexOf('/');
     if (indexOfSlash == -1) {
       throw new IllegalArgumentException("Invalid mime type: " + mimeType);