fixed segfault when screen number exceeds screen count (#2534)

src/core/flameshot.cpp

@@ -132,6 +132,7 @@ CaptureWidget* Flameshot::gui(const CaptureRequest& req)
         return m_captureWindow;
     } else {
         emit captureFailed();
+        return nullptr;
     }
 }
 
@@ -151,6 +152,11 @@ void Flameshot::screen(CaptureRequest req, const int screenNumber)
         screen =
           qApp->screens()[qApp->desktop()->screenNumber(globalCursorPos)];
 #endif
+    } else if (screenNumber >= qApp->screens().count()) {
+        AbstractLogger() << QObject::tr(
+          "Requested screen exceeds screen count");
+        emit captureFailed();
+        return;
     } else {
         screen = qApp->screens()[screenNumber];
     }

src/main.cpp

@@ -442,7 +442,7 @@ int main(int argc, char* argv[])
 
         QString numberStr = parser.value(screenNumberOption);
         // Option values
-        int number =
+        int screenNumber =
           numberStr.startsWith(QLatin1String("-")) ? -1 : numberStr.toInt();
         QString path = parser.value(pathOption);
         if (!path.isEmpty()) {
@@ -455,7 +455,7 @@ int main(int argc, char* argv[])
         bool pin = parser.isSet(pinOption);
         bool upload = parser.isSet(uploadOption);
 
-        CaptureRequest req(CaptureRequest::SCREEN_MODE, delay, number);
+        CaptureRequest req(CaptureRequest::SCREEN_MODE, delay, screenNumber);
         if (!region.isEmpty()) {
             if (region.startsWith("screen")) {
                 // TODO use abstract logger