Small sftp-common.c refactoring

[pmeinhardt]

Make the attrib-stat conversion functions more consistent.

1. Extract stat_clear() function

   Make stat struct clearing consistent with how attributes are cleared, using the attrib_clear() function.

2. Clear attributes using memset(), same as stats

   We are using memset() to clear stats. We can use the same approach to clear attribute structs.

[pmeinhardt]

I branched off of master and the master branch is red, which is probably what causes the build for this PR to fail. 🚥 I could rebase onto the last "green" commit to make this PR pass. What do you think?

[pmeinhardt]

Looks like this is only failing because the master branch is "red". How do you suggest to proceed in this case?