libmount: (mountinfo) parse empty strings in source

libmount: (mountinfo) parse empty strings in source

The source of a mount in /proc/self/mountinfo can unfortunately be an
empty string. Before this patch, 'mount' and 'mountpoint' fail as
following:

  $ sudo mount -t tmpfs "" /tmp/bb
  $ mount
  mount: /proc/self/mountinfo: parse error at line 64 -- ignored
  $ mountpoint /tmp/bb
  /tmp/bb is not a mountpoint

This patch fixes the parsing. It is unfortunately more complex because
sscanf() does not handle fields with empty strings easily.

Other projects have their own parser for mountinfo and have similar
issues. I know of runc and runtime-tools (I'll send a patch for those
two) but there are probably others.

Signed-off-by: Alban Crequy <alban@kinvolk.io>