SDL_FindFreePlayerIndex() will always return the next available player index

Fixes https://github.com/libsdl-org/SDL/issues/7868

src/joystick/SDL_joystick.c

@@ -218,10 +218,10 @@ static int SDL_FindFreePlayerIndex(void)
 
     for (player_index = 0; player_index < SDL_joystick_player_count; ++player_index) {
         if (SDL_joystick_players[player_index] == 0) {
-            return player_index;
+            break;
         }
     }
-    return -1;
+    return player_index;
 }
 
 static int SDL_GetPlayerIndexForJoystickID(SDL_JoystickID instance_id)