LibCrypto: Do not silently ignore key size mismatch

Before, when the actually passed key was too long, the extra bytes were silently
ignored. This can lead to all sorts of trouble, so ... don't do that.

The original intention was maybe to support non-integer amounts of key bytes.
But that doesn't happen anyway with AES.

Libraries/LibCrypto/Cipher/AES.cpp

@@ -67,7 +67,7 @@ void AESCipherKey::expand_encrypt_key(const ByteBuffer& user_key, size_t bits)
 
     ASSERT(!user_key.is_null());
     ASSERT(is_valid_key_size(bits));
-    ASSERT(user_key.size() >= bits / 8);
+    ASSERT(user_key.size() == bits / 8);
 
     round_key = round_keys();