A conversation can't be trimmed to 0.

src/org/thoughtcrime/securesms/ApplicationPreferencesActivity.java

@@ -383,6 +383,10 @@ public class ApplicationPreferencesActivity extends SherlockPreferenceActivity {
         return false;
       }
       
+      if (Integer.parseInt((String)newValue) < 1) {
+        return false;
+      }
+
       preference.setSummary((String)newValue + " " +
                             getString(R.string.ApplicationPreferencesActivity_messages_per_conversation));
       return true;