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

Probable regression introduced in commit
02ab29d2f1 (1.13.14).

changelog.md

@@ -1,6 +1,10 @@
 ## Version 1.13.14+dev
  ### Language and i18n
    * Updated translations: French, Galician
+ ### Multiplayer
+   * Fixed regression causing a crash-to-desktop when trying to log into the
+     server using a registered and active account without specifying a
+     password.
  ### User Interface
    * Implemented MP chat message history saving (issue #1194, issue #2802).
  ### Miscellaneous and bug fixes

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;