[Backport r42951 | guido.van.rossum]

[Backport r42951 | guido.van.rossum]

Fix three nits found by Coverity, adding null checks and comments.

[This commit only makes two changes.  One change in the original patch
 is just adding a comment, and another adds a 'base != NULL' check to
 silence Coverity, but a comment adds that that base is never going to
 be NULL.  I didn't backport that change. --amk]