Linux: Fix a out-of-bounds memory write when copying text to the clipboard

modules/juce_gui_basics/native/x11/juce_linux_XWindowSystem.cpp

@@ -1361,8 +1361,10 @@ namespace ClipboardHelpers
             {
                 // another application wants to know what we are able to send
                 numDataItems = 2;
-                propertyFormat = 32; // atoms are 32-bit
-                data.calloc (numDataItems * 4);
+                constexpr size_t atomSize = sizeof (Atom);
+                static_assert (atomSize == 8, "Atoms are 32-bit");
+                propertyFormat = atomSize * 4;
+                data.calloc (numDataItems * atomSize);
 
                 auto* dataAtoms = unalignedPointerCast<Atom*> (data.getData());