Do not cache atom name if XInternAtom fails

If `XInternAtom` is called with `onlyIfExists` set, then it can
legitimately return `None`. We should not cache this value because it
might change in the future.

This bug can sometimes be triggered because we use
`getAtom("UTF8_STRING", true)` and `getAtom("UTF8_STRING")`. If the
first call caches `None` because the atom didn't exist, then the
second call could return `None` instead of creating a new atom like it
should.

src/SFML/Window/Unix/Display.cpp

@@ -129,6 +129,7 @@ Atom getAtom(const std::string& name, bool onlyIfExists)
 
     const auto display = openDisplay();
     const Atom atom    = XInternAtom(display.get(), name.c_str(), onlyIfExists ? True : False);
+    if (atom)
         atoms[name] = atom;
 
     return atom;