searchbar: fix warnings for invalid regular expressions

Certain characters such as @ or < were being treated as invalid
regular expressions and caused warnings to be raised. Added check
for validity of expression.

GitLab: #1628

Change-Id: I8b66ebfcf029cd0568bccdcba96672d9005846a9

src/app/conversationlistmodel.cpp

@@ -143,7 +143,7 @@ ConversationListProxyModel::filterAcceptsRow(int sourceRow, const QModelIndex& s
         }
     } else {
         Q_FOREACH (const auto& filter, toFilter)
-            if (rx.match(filter).hasMatch()) {
+            if (rx.isValid() && rx.match(filter).hasMatch()) {
                 match = true;
                 break;
             }