Fix wrong step size in wxImage::Paste()

Fix a bug with wrong size passed to memset() introduced in 1f0ade29f0
(Fix using mask colour even if there is no mask in wxImage::Paste,
2020-09-30) which caused memory corruption and add a test (the one with
the large negative vertical offset) allowing to reproduce this reliably.

Closes https://github.com/wxWidgets/wxWidgets/pull/2067

src/common/image.cpp

@@ -1764,7 +1764,7 @@ wxImage::Paste(const wxImage & image, int x, int y,
                 // Make all the copied pixels fully opaque
                 if (alpha_target_data != NULL)
                 {
-                    memset(alpha_target_data, wxALPHA_OPAQUE, target_alpha_step);
+                    memset(alpha_target_data, wxALPHA_OPAQUE, width);
                     alpha_target_data += target_alpha_step;
                 }
             }