Robustness fixes for WebM extractor.

- readFully calls when reading a string or varint may have 0 length.
  The behavior of a 0 length read isn't well defined either at the
  ExtractorInput or DataSource level, particularly when the read may
  also coincide with EOS. We'll work on defining these cases properly
  going forward, but in the meantime this fix avoids attempting 0
  length reads.
- [Aside] UTF8 the is guaranteed default charset on Android.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=117130356

library/src/main/java/com/google/android/exoplayer/extractor/webm/DefaultEbmlReader.java

@@ -22,7 +22,6 @@ import com.google.android.exoplayer.util.Assertions;
 
 import java.io.EOFException;
 import java.io.IOException;
-import java.nio.charset.Charset;
 import java.util.Stack;
 
 /**
@@ -215,9 +214,12 @@ import java.util.Stack;
    */
   private String readString(ExtractorInput input, int byteLength)
       throws IOException, InterruptedException {
+    if (byteLength == 0) {
+      return "";
+    }
     byte[] stringBytes = new byte[byteLength];
     input.readFully(stringBytes, 0, byteLength);
-    return new String(stringBytes, Charset.forName(C.UTF8_NAME));
+    return new String(stringBytes);
   }
 
   /**

library/src/main/java/com/google/android/exoplayer/extractor/webm/VarintReader.java

@@ -84,8 +84,10 @@ import java.io.IOException;
       return C.RESULT_MAX_LENGTH_EXCEEDED;
     }
 
-    // Read the remaining bytes.
-    input.readFully(scratch, 1, length - 1);
+    if (length != 1) {
+      // Read the remaining bytes.
+      input.readFully(scratch, 1, length - 1);
+    }
 
     state = STATE_BEGIN_READING;
     return assembleVarint(scratch, length, removeLengthMask);