mesa: fix GL_FRAMEBUFFER_ATTACHMENT_OBJECT_NAME query

Return 0 instead of generating an error.

See http://bugs.freedesktop.org/show_bug.cgi?id=30993

Note that piglit fbo-getframebufferattachmentparameter-01 still does
not pass.  But Mesa behaves the same as the NVIDIA driver in this case.
Perhaps the test is incorrect.

NOTE: This is a candidate for the 7.9 branch.

src/mesa/main/fbobject.c

@@ -2024,8 +2024,8 @@ _mesa_GetFramebufferAttachmentParameterivEXT(GLenum target, GLenum attachment,
 	 *params = att->Texture->Name;
       }
       else {
-	 _mesa_error(ctx, GL_INVALID_ENUM,
-		     "glGetFramebufferAttachmentParameterivEXT(pname)");
+         assert(att->Type == GL_NONE);
+         *params = 0;
       }
       return;
    case GL_FRAMEBUFFER_ATTACHMENT_TEXTURE_LEVEL_EXT: