Fix SIGSEGV on double free for Cred object

This change removes the Go finalizer when passing ownership to libgit2.

Fixes: #553

remote.go

@@ -255,6 +255,10 @@ func credentialsCallback(_cred **C.git_cred, _url *C.char, _username_from_url *C
 	}
 	if cred != nil {
 		*_cred = cred.ptr
+
+		// have transferred ownership to libgit, 'forget' the native pointer
+		cred.ptr = nil
+		runtime.SetFinalizer(cred, nil)
 	}
 	return 0
 }