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>
Reviewed-by: Daniel Stone <daniels@collabora.com>
diff --git a/libweston/compositor-fbdev.c b/libweston/compositor-fbdev.c
index 6a30538..b1cc5dc 100644
--- a/libweston/compositor-fbdev.c
+++ b/libweston/compositor-fbdev.c
@@ -383,6 +383,7 @@
 	if (output->fb == MAP_FAILED) {
 		weston_log("Failed to mmap frame buffer: %s\n",
 		           strerror(errno));
+		output->fb = NULL;
 		goto out_close;
 	}