Ensure bitmap image exists while querying for a row stride

Obtaining bytes per row makes sense only for bitmap image representation
so we have to ensure that the bitmap represenation exists before querying
for its row stride length. NSImage per se cannot provide this information.

Closes #18606.

src/osx/core/bitmap.cpp

@@ -324,14 +324,14 @@ int wxBitmapRefData::GetDepth() const
     else
         return 32; // a bitmap converted from an nsimage would have this depth
 }
+
 int wxBitmapRefData::GetBytesPerRow() const
 {
     wxCHECK_MSG( IsOk() , 0 , "Invalid Bitmap");
 
-    if ( m_hBitmap )
-        return (int) CGBitmapContextGetBytesPerRow(m_hBitmap);
-    else
-        return (int) GetBestBytesPerRow( GetWidth() * 4);
+    // Row stride length makes sense only for a bitmap representation
+    EnsureBitmapExists();
+    return (int) CGBitmapContextGetBytesPerRow(m_hBitmap);
 }
 
 bool wxBitmapRefData::HasAlpha() const