blockd: fix segfault triggered by non-autofs mounts

Program received signal SIGSEGV, Segmentation fault.
main_autofs (argv=<optimized out>, argc=<optimized out>)
    at fstools-2020-05-06-eec16e2f/block.c:1193
1193:    if (!m->autofs && (mp = find_mount_point(pr->dev))) {

Fixes: 9ab936d ("block(d): always call hotplug.d "mount" scripts from blockd")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>

block.c

@@ -1190,7 +1190,7 @@ static int main_autofs(int argc, char **argv)
 				continue;
 
 			blockd_notify("hotplug", pr->dev, m, pr);
-			if (!m->autofs && (mp = find_mount_point(pr->dev))) {
+			if ((!m || !m->autofs) && (mp = find_mount_point(pr->dev))) {
 				blockd_notify("mount", pr->dev, NULL, NULL);
 				free(mp);
 			}