block: Fix potential Null pointer dereferences in vvfat.c

block: Fix potential Null pointer dereferences in vvfat.c

The calls to find_mapping_for_cluster() may return NULL but it
isn't always checked for before dereferencing the value returned.
Additionally, add some asserts to cover cases where NULL can't
be returned but which might not be obvious at first glance.

Signed-off-by: Liam Merwick <Liam.Merwick@oracle.com>
Message-id: 1541453919-25973-5-git-send-email-Liam.Merwick@oracle.com
[mreitz: Dropped superfluous check of "mapping" following an assertion
         that it is not NULL, and fixed some indentation]
Signed-off-by: Max Reitz <mreitz@redhat.com>