compositor-fbdev: MAP_FAILED is not NULL

Fix the assumption that MAP_FAILED would be equal to NULL. It is not.
Set 'fb' explicitly to NULL on mmap failure so that comparisons to NULL
would produce the expected result.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

libweston/compositor-fbdev.c

@@ -381,6 +381,7 @@ fbdev_frame_buffer_map(struct fbdev_output *output, int fd)
 	if (output->fb == MAP_FAILED) {
 		weston_log("Failed to mmap frame buffer: %s\n",
 		           strerror(errno));
+		output->fb = NULL;
 		goto out_close;
 	}