input OSD: fix potentially absent P0

I don't know if there's any core that lacks non-player controls but this feels like the right thing to do anyway (for the future too)

src/BizHawk.Client.Common/display/InputDisplayGenerator.cs

@@ -16,10 +16,13 @@ public static string Generate(IController source)
 				throw new InvalidOperationException("Can't generate input display string with empty mnemonics cache");
 
 			var sb = new StringBuilder();
-			var controls = source.Definition.ControlsOrdered.ToList();
-			// index 0 is for controls that don't belong to any indexed player
-			controls.Add(controls[0]);
-			controls.RemoveAt(0);
+			var controls = source.Definition.ControlsOrdered.ToList();
+			// index 0 is for controls that don't belong to any indexed player
+			if (controls.ElementAtOrDefault(0) != null)
+			{
+				controls.Add(controls[0]);
+				controls.RemoveAt(0);
+			}
 
 			foreach ((string buttonName, AxisSpec? axisSpec) in controls.SelectMany(x => x))
 			{