unshare: make sure map_range.next is initialized [coverity scan]

unshare: make sure map_range.next is initialized [coverity scan]

* initialize during declaration (if possible), unspecified fields
  defaults to zero in this case.

* fix coverity warnings:
 CID 410290:  Uninitialized variables  (UNINIT)
 Using uninitialized value "mid". Field "mid.next" is uninitialized when calling "insert_map_range".

 CID 410289:  Uninitialized variables  (UNINIT)
 Using uninitialized value "hi". Field "hi.next" is uninitialized when calling "insert_map_range".

 CID 410288:  Uninitialized variables  (UNINIT)
 Using uninitialized value "lo". Field "lo.next" is uninitialized when calling "insert_map_range".

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