Fix MP4+MKV sniffing to handle empty atoms / EBML elements.

Issue: #641

library/src/main/java/com/google/android/exoplayer/extractor/mp4/Sniffer.java

@@ -102,7 +102,7 @@ import java.io.IOException;
         atomSize = buffer.readLong();
       }
       // Check the atom size is large enough to include its header.
-      if (atomSize <= headerSize || atomSize > Integer.MAX_VALUE) {
+      if (atomSize < headerSize || atomSize > Integer.MAX_VALUE) {
         return false;
       }
       // Stop searching if reading this atom would exceed the search limit.
@@ -129,7 +129,7 @@ import java.io.IOException;
       } else if (atomType == Atom.TYPE_moof) {
         foundFragment = true;
         break;
-      } else {
+      } else if (atomDataSize != 0) {
         input.advancePeekPosition(atomDataSize);
       }
       bytesSearched += atomSize;

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

@@ -76,11 +76,13 @@ import java.io.IOException;
         return false;
       }
       long size = readUint(input);
-      if (size <= 0 || size > Integer.MAX_VALUE) {
+      if (size < 0 || size > Integer.MAX_VALUE) {
         return false;
       }
-      input.advancePeekPosition((int) size);
-      peekLength += size;
+      if (size != 0) {
+        input.advancePeekPosition((int) size);
+        peekLength += size;
+      }
     }
     return peekLength == headerStart + headerSize;
   }

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

@@ -1187,7 +1187,8 @@ public final class WebmExtractor implements Extractor {
             System.arraycopy(NalUnitUtil.NAL_START_CODE, 0, buffer, bufferPosition,
                 NalUnitUtil.NAL_START_CODE.length);
             bufferPosition += NalUnitUtil.NAL_START_CODE.length;
-            System.arraycopy(parent.data, parent.getPosition(), buffer, bufferPosition, nalUnitLength);
+            System.arraycopy(parent.data, parent.getPosition(), buffer, bufferPosition,
+                nalUnitLength);
             bufferPosition += nalUnitLength;
             parent.skipBytes(nalUnitLength);
           }