Suppress static analyzer warning in sa_attr_iter()

Clang's static analyzer points out that when IS_SA_BONUSTYPE(type) is
true and .sa_length is 0 for an attribute, we have a NULL pointer
dereference. We suppress this with an IMPLY() statement.

This was also identified by Coverity.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reported-by: Coverity (CID-1017954)
Closes #14470