PAM: Fix uninitialized value read

Clang's static analyzer found that config.uid is uninitialized when
zfs_key_config_load() returns an error.

Oddly, this was not included in the unchecked return values that
Coverity found.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #13957