[COVERITY] Check for NULL return from dict_lookup() in e2fsck

[COVERITY] Check for NULL return from dict_lookup() in e2fsck

The dict_lookup() function can potentially return a NULL dnode_t.  It is
not checked in two places in the clone_file() function.  Looks to be
safe to continue if n is NULL, so just print a warning message and
continue.

Coverity ID: 9: Null Returns

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>