Fixed error return values of native EGL functions.

src/egl_context.c

@@ -513,21 +513,21 @@ GLFWglproc _glfwPlatformGetProcAddress(const char* procname)
 
 GLFWAPI EGLDisplay glfwGetEGLDisplay(void)
 {
-    _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
+    _GLFW_REQUIRE_INIT_OR_RETURN(EGL_NO_DISPLAY);
     return _glfw.egl.display;
 }
 
 GLFWAPI EGLContext glfwGetEGLContext(GLFWwindow* handle)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
-    _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
+    _GLFW_REQUIRE_INIT_OR_RETURN(EGL_NO_CONTEXT);
     return window->egl.context;
 }
 
 GLFWAPI EGLSurface glfwGetEGLSurface(GLFWwindow* handle)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
-    _GLFW_REQUIRE_INIT_OR_RETURN(0);
+    _GLFW_REQUIRE_INIT_OR_RETURN(EGL_NO_SURFACE);
     return window->egl.surface;
 }