Fix buffer overrun in wxHTMLDataObject under non-MSW platforms

Using strcpy() in GetDataHere() added an extra NUL at the end which
didn't fit into the buffer of the size returned by GetDataSize(). This
could have been also fixed by returning an extra byte from the latter
function, but as the string doesn't need to be NUL-terminated,
apparently, just use memcpy() with the correct number of bytes instead.

Also, because the string is not necessarily NUL-terminated, use the
provided length in wxHTMLDataObject::SetData() instead of relying on the
buffer being NUL-terminated and reading uninitialized memory beyond its
size.

Add a unit test confirming that there are no more ASAN errors when using
this class.

Closes #23660.

Co-Authored-By: mcorino <martin@corino.nl>

tests/misc/guifuncs.cpp

@@ -92,6 +92,22 @@ TEST_CASE("GUI::URLDataObject", "[guifuncs][clipboard]")
     CHECK( dobj2.GetURL() == url );
 }
 
+TEST_CASE("GUI::HTMLDataObject", "[guifuncs][clipboard]")
+{
+    const wxString text("<h1>Hello clipboard!</h1>");
+
+    wxHTMLDataObject* const dobj = new wxHTMLDataObject(text);
+    CHECK( dobj->GetHTML() == text );
+
+    wxClipboardLocker lockClip;
+    CHECK( wxTheClipboard->SetData(dobj) );
+    wxTheClipboard->Flush();
+
+    wxHTMLDataObject dobj2;
+    REQUIRE( wxTheClipboard->GetData(dobj2) );
+    CHECK( dobj2.GetHTML() == text );
+}
+
 // This disabled by default test allows to check that we retrieve HTML data
 // from the system clipboard correctly.
 TEST_CASE("GUI::ShowHTML", "[.]")