Discard subtitles with invalid positions

textWidth can be negative if textLeft extends parentRight (i.e.
the subtitle is positioned entirely off the screen to the RHS).
We should just discard and log a warning in this case.

Issue: #2497

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=148481310

library/src/main/java/com/google/android/exoplayer2/ui/SubtitlePainter.java

@@ -266,6 +266,12 @@ import com.google.android.exoplayer2.util.Util;
       textRight = textLeft + textWidth;
     }
 
+    textWidth = textRight - textLeft;
+    if (textWidth <= 0) {
+      Log.w(TAG, "Skipped drawing subtitle cue (invalid horizontal positioning)");
+      return;
+    }
+
     int textTop;
     if (cueLine != Cue.DIMEN_UNSET) {
       int anchorPosition;
@@ -292,8 +298,6 @@ import com.google.android.exoplayer2.util.Util;
       textTop = parentBottom - textHeight - (int) (parentHeight * bottomPaddingFraction);
     }
 
-    textWidth = textRight - textLeft;
-
     // Update the derived drawing variables.
     this.textLayout = new StaticLayout(cueText, textPaint, textWidth, textAlignment, spacingMult,
         spacingAdd, true);