Fixed unguarded calls to monitor callback.

README.md

@@ -461,6 +461,7 @@ skills.
  * Matt Arsenault
  * Keith Bauer
  * John Bartholomew
+ * Niklas Bergström
  * blanco
  * Lambert Clara
  * Noel Cower

src/monitor.c

@@ -140,6 +140,7 @@ void _glfwInputMonitorChange(void)
                 window->monitor = NULL;
         }
 
+        if (_glfw.monitorCallback)
             _glfw.monitorCallback((GLFWmonitor*) monitors[i], GLFW_DISCONNECTED);
     }
 
@@ -161,6 +162,7 @@ void _glfwInputMonitorChange(void)
         if (j < monitorCount)
             continue;
 
+        if (_glfw.monitorCallback)
             _glfw.monitorCallback((GLFWmonitor*) _glfw.monitors[i], GLFW_CONNECTED);
     }