writen to the odb

libgit2 has now the same behaviour of git when adding a tag with a
buffer.

This commit create a function called tag_valid_in_odb which validate a
tag before its creation. This function will be needed by my next commit.

When a tag already exists, it can be useful to directly have the oid
of the existed tag, just after trying to add it.

Before this commit, no message is shown when doing a git_lasterror().

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.