Fix crash when using custom DnD formats under Wine

Using ::HeapSize() on a global pointer is wrong, and even though it
somehow still works under "genuine" MSW, it crashes under Wine.

Fix this by using ::GlobalSize() instead, which is the right function to
use with this kind of pointer.

Thanks to Damjan Jovanovic for the analysis of the problem in Wine
bugzilla (see https://bugs.winehq.org/show_bug.cgi?id=38924#c10).

Closes #18887.

src/msw/ole/dataobj.cpp

@@ -975,14 +975,10 @@ const void *wxDataObject::GetSizeFromBuffer(const void *buffer,
                                             size_t *size,
                                             const wxDataFormat& WXUNUSED(format))
 {
-    // hack: the third parameter is declared non-const in Wine's headers so
-    // cast away the const
-    const size_t realsz = ::HeapSize(::GetProcessHeap(), 0,
-                                     const_cast<void*>(buffer));
-    if ( realsz == (size_t)-1 )
+    const size_t realsz = ::GlobalSize(::GlobalHandle(buffer));
+    if ( !realsz )
     {
-        // note that HeapSize() does not set last error
-        wxLogApiError(wxT("HeapSize"), 0);
+        wxLogLastError(wxT("GlobalSize"));
         return NULL;
     }