keychain: Pad user name if its read out from keychain

Tests show that the username actually still needs to be padded.
Otherwise, crashes can occur in subsequent usage of username, as
it might contain garbage.

modules/keystore/keychain.m

@@ -399,7 +399,10 @@ static unsigned int Find(vlc_keystore *p_keystore,
                 case kSecAccountItemAttr:
                     if (!p_entry->ppsz_values[KEY_USER]) {
                         msg_Dbg(p_keystore, "using account name from the keychain for login");
-                        p_entry->ppsz_values[KEY_USER] = strdup((const char *)attr->data);
+
+                        char *paddedName = calloc(1, attr->length + 1);
+                        memcpy(paddedName, attr->data, attr->length);
+                        p_entry->ppsz_values[KEY_USER] = paddedName;
                     }
                     break;
                 default: