stransport: do not use `git_stream_free` on uninitialized stransport
stransport: do not use `git_stream_free` on uninitialized stransport

When failing to initialize a new stransport stream, we try to
release already allocated memory by calling out to
`git_stream_free`, which in turn called out to the stream's
`free` function pointer. As we only initialize the function
pointer later on, this leads to a `NULL` pointer exception.

Furthermore, plug another memory leak when failing to create the
SSL context.

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