Fix segfault when trying to log into the MP server with an empty password

Probable regression introduced in commit
02ab29d2f1 (1.13.14).

(cherry-picked from commit fc88cdff3b)

src/preferences/credentials.cpp

@@ -278,7 +278,7 @@ secure_buffer encrypt(const secure_buffer& text, const secure_buffer& key)
 secure_buffer decrypt(const secure_buffer& text, const secure_buffer& key)
 {
 	auto buf = rc4_crypt(text, key);
-	while(buf.back() == 0) {
+	while(!buf.empty() && buf.back() == 0) {
 		buf.pop_back();
 	}
 	return buf;