Be robust against unexpected EOS in WebvttCueParser

Issue: #3228

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=167504122

library/core/src/main/java/com/google/android/exoplayer2/text/webvtt/WebvttCueParser.java

@@ -21,6 +21,7 @@ import android.text.Layout.Alignment;
 import android.text.Spannable;
 import android.text.SpannableStringBuilder;
 import android.text.Spanned;
+import android.text.TextUtils;
 import android.text.style.AbsoluteSizeSpan;
 import android.text.style.AlignmentSpan;
 import android.text.style.BackgroundColorSpan;
@@ -92,20 +93,25 @@ import java.util.regex.Pattern;
   /* package */ boolean parseCue(ParsableByteArray webvttData, WebvttCue.Builder builder,
       List<WebvttCssStyle> styles) {
     String firstLine = webvttData.readLine();
+    if (firstLine == null) {
+      return false;
+    }
     Matcher cueHeaderMatcher = WebvttCueParser.CUE_HEADER_PATTERN.matcher(firstLine);
     if (cueHeaderMatcher.matches()) {
       // We have found the timestamps in the first line. No id present.
       return parseCue(null, cueHeaderMatcher, webvttData, builder, textBuilder, styles);
-    } else {
+    }
     // The first line is not the timestamps, but could be the cue id.
     String secondLine = webvttData.readLine();
+    if (secondLine == null) {
+      return false;
+    }
     cueHeaderMatcher = WebvttCueParser.CUE_HEADER_PATTERN.matcher(secondLine);
     if (cueHeaderMatcher.matches()) {
       // We can do the rest of the parsing, including the id.
       return parseCue(firstLine.trim(), cueHeaderMatcher, webvttData, builder, textBuilder,
           styles);
     }
-    }
     return false;
   }
 
@@ -233,7 +239,7 @@ import java.util.regex.Pattern;
     // Parse the cue text.
     textBuilder.setLength(0);
     String line;
-    while ((line = webvttData.readLine()) != null && !line.isEmpty()) {
+    while (!TextUtils.isEmpty(line = webvttData.readLine())) {
       if (textBuilder.length() > 0) {
         textBuilder.append("\n");
       }