Qt: Show capture filter warning

Change-Id: I080bcb25edcee0c38984cd1a7f7f36a7fd3ff36c
Reviewed-on: https://code.wireshark.org/review/13055
Reviewed-by: Stig Bjørlykke <stig@bjorlykke.org>

ui/qt/capture_filter_edit.cpp

@@ -310,7 +310,7 @@ void CaptureFilterEdit::setFilterSyntaxState(QString filter, int state, QString
 
     if (filter.compare(text()) == 0) { // The user hasn't changed the filter
         setSyntaxState((SyntaxState)state);
-        if (!valid) {
+        if (!err_msg.isEmpty()) {
             emit pushFilterSyntaxStatus(err_msg);
         }
     }