Add check for converting negative SDL_GameControllerButton to glyph

A SDL_GameControllerButton below 0 is out of array bounds, so that
should trip the assert and return GLYPH_UNKNOWN just like when the
value is too high.

Author: Daaaav

desktop_version/src/ButtonGlyphs.cpp

@@ -274,7 +274,7 @@ const char* BUTTONGLYPHS_get_wasd_text(void)
 
 const char* BUTTONGLYPHS_sdlbutton_to_glyph(const SDL_GameControllerButton button)
 {
-    if (button > SDL_CONTROLLER_BUTTON_RIGHTSHOULDER)
+    if (button < 0 || button > SDL_CONTROLLER_BUTTON_RIGHTSHOULDER)
     {
         SDL_assert(0 && "Unhandled button!");
         return glyph[GLYPH_UNKNOWN];