libmount: be robust for empty target/source strings

 * lib/canonicalize.c: don't interpret empty strings as relative paths
 * libmount: more robust libmnt_table find function and debug messages

References: https://bugzilla.novell.com/show_bug.cgi?id=825150
Signed-off-by: Karel Zak <kzak@redhat.com>

Signed-off-by: Karel Zak <kzak@redhat.com>