exit(3), don't crash, when glfwCreateWindow fails

glfwSetWindowPos was called on a NULL window, triggering an assert
inside GLFW. Check for failure and exit cleanly by means of
TraceLog(LOG_ERROR instead.

src/core.c

@@ -1822,6 +1822,8 @@ static void InitGraphicsDevice(int width, int height)
         // No-fullscreen window creation
         window = glfwCreateWindow(screenWidth, screenHeight, windowTitle, NULL, NULL);
 
+        if (window)
+        {
 #if defined(PLATFORM_DESKTOP)
             // Center window on screen
             int windowPosX = displayWidth/2 - screenWidth/2;
@@ -1835,6 +1837,7 @@ static void InitGraphicsDevice(int width, int height)
             renderWidth = screenWidth;
             renderHeight = screenHeight;
         }
+    }
 
     if (!window)
     {