limit passphrase length to space for 250 code points in UTF-8

passphrase_cache.cc

@@ -6,7 +6,7 @@ namespace pEp {
         std::lock_guard<std::mutex> lock(_mtx);
         while (_cache.size() >= _max_size)
             _cache.pop_front();
-        _cache.push_back({passphrase, clock::now()});
+        _cache.emplace_back(cache_entry(passphrase, clock::now()));
     }
 
     bool PassphraseCache::for_each_passphrase(const passphrase_callee& callee)

passphrase_cache.hh

@@ -13,6 +13,10 @@ namespace pEp {
         using duration = clock::duration;
 
         struct cache_entry {
+            static const size_t max_len = 250 * 4;
+            cache_entry(std::string p, time_point t) :
+                passphrase(p, 0, max_len), tp(t) { }
+
             std::string passphrase;
             time_point tp;
         };