global: adjust init count under lock

Our global initialization functions `git_libgit2_init()` and
`git_libgit2_shutdown()` both adjust a global init counter to determine
whether we are the first respectively last user of libgit2. On
Unix-systems do not do so under lock, though, which opens the
possibility of a race between these two functions:

    Thread 1                            Thread 2
                            git__n_inits = 0;
    git_libgit2_init();
    git_atomic_inc(&git__n_inits);
    /* git__n_inits == 1 */
                                        git_libgit2_shutdown();
                                        if (git_atomic_dec(&git__n_inits) != 0)
                                            /* git__n_inits == 0, no early exit here */
                                        pthread_mutex_lock(&_init_mutex);
                                        shutdown_common();
                                        pthread_mutex_unlock(&_init_mutex);
    pthread_mutex_lock(&_init_mutex);
    init_once();
    pthread_mutex_unlock(&_init_mutex);

So we can end up in a situation where we try to shutdown shared data
structures before they have been initialized.

Fix the race by always locking `_init_mutex` before incrementing or
decrementing `git__n_inits`.

src/global.c

@@ -272,10 +272,10 @@ int git_libgit2_init(void)
 {
 	int ret, err;
 
-	ret = git_atomic_inc(&git__n_inits);
-
 	if ((err = pthread_mutex_lock(&_init_mutex)) != 0)
 		return err;
+
+	ret = git_atomic_inc(&git__n_inits);
 	err = pthread_once(&_once_init, init_once);
 	err |= pthread_mutex_unlock(&_init_mutex);
 
@@ -289,13 +289,13 @@ int git_libgit2_shutdown(void)
 {
 	void *ptr = NULL;
 	pthread_once_t new_once = PTHREAD_ONCE_INIT;
-	int ret;
+	int error, ret;
 
-	if ((ret = git_atomic_dec(&git__n_inits)) != 0)
-		return ret;
+	if ((error = pthread_mutex_lock(&_init_mutex)) != 0)
+		return error;
 
-	if ((ret = pthread_mutex_lock(&_init_mutex)) != 0)
-		return ret;
+	if ((ret = git_atomic_dec(&git__n_inits)) != 0)
+		goto out;
 
 	/* Shut down any subsystems that have global state */
 	shutdown_common();
@@ -310,10 +310,11 @@ int git_libgit2_shutdown(void)
 	git_mutex_free(&git__mwindow_mutex);
 	_once_init = new_once;
 
-	if ((ret = pthread_mutex_unlock(&_init_mutex)) != 0)
-		return ret;
+out:
+	if ((error = pthread_mutex_unlock(&_init_mutex)) != 0)
+		return error;
 
-	return 0;
+	return ret;
 }
 
 git_global_st *git__global_state(void)