Before this commit, malformed tag and signature were considered as
valid by the parser. See the test t3800-mktag.sh of git to see example
of malformed tag and signature.

Fix compilation warnings in MSVC

This allows to successfully build libgit2 with waf on Windows.

When trying to inflate a buffer, a GIT_ERROR was returned when malloc()
failed.

Fix this to return GIT_ENOMEM.

git__delta_apply used to return with a generic GIT_ERROR when allocating
memory for the delta failed.

Fix this to return GIT_ENOMEM.