Bitcoin-Qt: extend parseBitcoinURI() pre-check

- add check to verify if an URI is valid

src/qt/guiutil.cpp

@@ -77,7 +77,8 @@ void setupAmountWidget(QLineEdit *widget, QWidget *parent)
 
 bool parseBitcoinURI(const QUrl &uri, SendCoinsRecipient *out)
 {
-    if(uri.scheme() != QString("bitcoin"))
+    // return if URI is not valid or is no bitcoin URI
+    if(!uri.isValid() || uri.scheme() != QString("bitcoin"))
         return false;
 
     SendCoinsRecipient rv;