Fix crash when processing invalid mentions.

app/src/main/java/org/thoughtcrime/securesms/database/MentionUtil.java

@@ -76,6 +76,10 @@ public final class MentionUtil {
     int bodyIndex = 0;
 
     for (Mention mention : sortedMentions) {
+      if (invalidMention(body, mention)) {
+        return new UpdatedBodyAndMentions(body, Collections.emptyList());
+      }
+
       updatedBody.append(body.subSequence(bodyIndex, mention.getStart()));
       CharSequence replaceWith    = replacementTextGenerator.apply(mention);
       Mention      updatedMention = new Mention(mention.getRecipientId(), updatedBody.length(), replaceWith.length());
@@ -139,6 +143,13 @@ public final class MentionUtil {
     throw new IllegalArgumentException("Unknown mention setting: " + mentionSetting);
   }
 
+  private static boolean invalidMention(@NonNull CharSequence body, @NonNull Mention mention) {
+    int start  = mention.getStart();
+    int length = mention.getLength();
+
+    return start < 0 || length < 0 || (start + length) > body.length();
+  }
+
   public static class UpdatedBodyAndMentions {
     @Nullable private final CharSequence  body;
     @NonNull  private final List<Mention> mentions;

app/src/test/java/org/thoughtcrime/securesms/database/MentionUtilTest_updateBodyAndMentionsWithPlaceholders.java

@@ -69,6 +69,8 @@ public class MentionUtilTest_updateBodyAndMentionsWithPlaceholders {
       builder().text("middle 🤡👍🏾 👨🏼‍🤝‍👨🏽 ").mention("a").text(" 👍🏾 middle 👩‍👩‍👦‍👦").build(),
       builder().text("start ").mention("emoji 🩳").build(),
       builder().text("start ").mention("emoji 🩳").text(" middle ").mention("emoji 🩳").text(" end").build(),
+
+      { "message", Collections.singletonList(new Mention(RecipientId.from(1), 30, 5)), "message", Collections.<Mention>emptyList() }
     });
   }