Bug: Segfault state_history_plugin when fetch_block: true

[vtsurka]

I run node from snapshot and exists state-history. If in get_blocks_request_v0 we set fetch_block: true it will be throw segfault.
Code from state_history_plugin.cpp

   void get_block(uint32_t block_num, fc::optional<bytes>& result) {
      chain::signed_block_ptr p;
      try {
         p = chain_plug->chain().fetch_block_by_number(block_num);
      } catch (...) {
         return;
      }
      result = fc::raw::pack(*p);
   }

I think we need to add condition on null result and maybe some warning log

if (p) {
      result = fc::raw::pack(*p);
}

[taokayan]

nice catch @vtsurka

[heifner]

Fixed by #7384