index_read_index: reset error correctly
index_read_index: reset error correctly

Clear any error state upon each iteration.  If one of the iterations
ends (with an error of `GIT_ITEROVER`) we need to reset that error to 0,
lest we stop the whole process prematurely.

stewid/libgit2,Corillian/libgit2,stewid/libgit2,Corillian/libgit2,Corillian/libgit2,Corillian/libgit2,stewid/libgit2,stewid/libgit2,stewid/libgit2,stewid/libgit2,Corillian/libgit2,Corillian/libgit2