ssl: init only once without threads

The OpenSSL library-loading functions do not expect to be called
multiple times. Add a flag in the non-threaded libgit2 init so we only
call once.

This fixes #2446.

src/global.c

@@ -291,7 +291,13 @@ static git_global_st __state;
 
 int git_threads_init(void)
 {
+	static int ssl_inited = 0;
+
+	if (!ssl_inited) {
 		init_ssl();
+		ssl_inited = 1;
+	}
+
 	git_atomic_inc(&git__n_inits);
 	return 0;
 }