parse_mountinfo_ent: xrealloc(new->mountpoint) can fail

This is pure theoretical, especially in this particular case when we
actually want to (likely) free the unused memory. Still the code which
ignores potential error doesn't look good.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>