Avoid crash in wxBitmap::GetImage() for invalid bitmaps

Just return NULL from this (wxOSX private, in spite of not using a
port-specific prefix) method.

This fixes crash in wxButton::SetBitmapXXX(wxNullBitmap), as shown by
the new test case which used to crash but doesn't do it any longer.

Closes #19257.

include/wx/osx/bitmap.h

@@ -204,6 +204,7 @@ public:
     // returns a CGImageRef which must released after usage with CGImageRelease
     CGImageRef CreateCGImage() const ;
 
+    // returns nil for invalid bitmap
     WXImage GetImage() const;
 #if wxOSX_USE_COCOA
     // returns an autoreleased version of the image

src/osx/core/bitmap.cpp

@@ -951,7 +951,7 @@ bool wxBitmap::Create(CGContextRef bitmapcontext)
 
 WXImage wxBitmap::GetImage() const
 {
-    return GetBitmapData()->GetImage();
+    return IsOk() ? GetBitmapData()->GetImage() : NULL;
 }
 
 wxBitmap wxBitmap::GetSubBitmap(const wxRect &rect) const

tests/controls/buttontest.cpp

@@ -160,6 +160,11 @@ TEST_CASE_METHOD(ButtonTestCase, "Button::Bitmap", "[button]")
     // updating the bitmap later, as it used to be the case in wxGTK (#18898).
     m_button->SetLabel(wxString());
     CHECK_NOTHROW( m_button->Disable() );
+
+    // Also check that setting an invalid bitmap doesn't do anything untoward,
+    // such as crashing, as it used to do in wxOSX (#19257).
+    CHECK_NOTHROW( m_button->SetBitmapPressed(wxNullBitmap) );
+    CHECK( !m_button->GetBitmapPressed().IsOk() );
 }
 
 #endif //wxUSE_BUTTON