Fix Issue #4047 Check return codes and free objects - #4056

While not exactly the same, this error in the appveyor log looks similar to the one I got for Pull Request #4052. In both cases a "threads::" test failed even though these tests do not invoke examples/general (at least that's my belief).

1: threads::iterator.
1: threads::refdb 1) Failure:
1: threads::refdb::edit_while_iterate [c:/projects/libgit2/tests/threads/refdb.c:130]
1: Function call failed: (error)
1: thread 0x88c - error -1 -
1:
1: 2) Failure:
1: threads::refdb::edit_while_iterate [c:/projects/libgit2/tests/clar/fs.h:76]
1: Expression is not true: DeleteFileW(buffer)
1:
1: Fatal error: a cleanup method raised an exception.
1/4 Test #1: libgit2_clar .............................***Failed 168.48 sec
test 2

This evening I reread the *.md documents on the libgit2 account, and I came upon the following sentence in CONTRIBUTING.md.

"If you use your fork's master branch directly, things can get messy."

I must have overlooked this sentence the first time I read these documents, because both my first Pull Request (#4052) and this one were for a commit on my fork's master branch. In order to avoid any messiness I am closing this Pull Request. I will resubmit it tomorrow using a temporary branch in my fork.