Fix empty container box bug

If a container box is empty, it is never removed
from the container box stack, breaking the extractor.

Issue: #1308
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=117228211

library/src/main/java/com/google/android/exoplayer/extractor/mp4/Mp4Extractor.java

@@ -195,7 +195,12 @@ public final class Mp4Extractor implements Extractor, SeekMap {
     if (shouldParseContainerAtom(atomType)) {
       long endPosition = input.getPosition() + atomSize - atomHeaderBytesRead;
       containerAtoms.add(new ContainerAtom(atomType, endPosition));
-      enterReadingAtomHeaderState();
+      if (atomSize == atomHeaderBytesRead) {
+        processAtomEnded(endPosition);
+      } else {
+        // Start reading the first child atom.
+        enterReadingAtomHeaderState();
+      }
     } else if (shouldParseLeafAtom(atomType)) {
       // We don't support parsing of leaf atoms that define extended atom sizes, or that have
       // lengths greater than Integer.MAX_VALUE.
@@ -238,7 +243,11 @@ public final class Mp4Extractor implements Extractor, SeekMap {
         seekRequired = true;
       }
     }
+    processAtomEnded(atomEndPosition);
+    return seekRequired && parserState != STATE_READING_SAMPLE;
+  }
 
+  private void processAtomEnded(long atomEndPosition) {
     while (!containerAtoms.isEmpty() && containerAtoms.peek().endPosition == atomEndPosition) {
       Atom.ContainerAtom containerAtom = containerAtoms.pop();
       if (containerAtom.type == Atom.TYPE_moov) {
@@ -246,14 +255,13 @@ public final class Mp4Extractor implements Extractor, SeekMap {
         processMoovAtom(containerAtom);
         containerAtoms.clear();
         parserState = STATE_READING_SAMPLE;
-        return false;
       } else if (!containerAtoms.isEmpty()) {
         containerAtoms.peek().add(containerAtom);
       }
     }
-
-    enterReadingAtomHeaderState();
-    return seekRequired;
+    if (parserState != STATE_READING_SAMPLE) {
+      enterReadingAtomHeaderState();
+    }
   }
 
   /**
@@ -289,7 +297,6 @@ public final class Mp4Extractor implements Extractor, SeekMap {
     if (udta != null) {
       gaplessInfo = AtomParsers.parseUdta(udta);
     }
-
     for (int i = 0; i < moov.containerChildren.size(); i++) {
       Atom.ContainerAtom atom = moov.containerChildren.get(i);
       if (atom.type != Atom.TYPE_trak) {