LibWeb: Don't try to deserialize past length of strings

If we serialize a string followed by some other object, the deserialize
helper would just happily keep appending bytes to the string until the
end of the serialization buffer. Avoid doing that by checking the string
length for figuring out when the string actually ends.

Userland/Libraries/LibWeb/HTML/StructuredSerialize.cpp

@@ -332,7 +332,7 @@ private:
         Vector<u8> bytes;
         TRY_OR_THROW_OOM(vm, bytes.try_ensure_capacity(size));
         u64 byte_position = 0;
-        while (position < vector.size()) {
+        while (position < vector.size() && byte_position < size) {
             for (u8 i = 0; i < 4; ++i) {
                 bytes.append(vector[position] >> (i * 8) & 0xFF);
                 byte_position++;