LibCrypto: Fix HMAC nullptr deref when key has length 0

While a key of length zero seems like a bad idea in general, the WPT
tests end up calling exactly this, so we need to support it one way or
another.

Userland/Libraries/LibCrypto/Authentication/HMAC.h

@@ -93,7 +93,7 @@ private:
             auto digest = m_inner_hasher.digest();
             // FIXME: should we check if the hash function creates more data than its block size?
             key_buffer.overwrite(0, digest.immutable_data(), m_inner_hasher.digest_size());
-        } else {
+        } else if (length > 0) {
             key_buffer.overwrite(0, key, length);
         }