wallet2: fix infinite loop on future refresh height

If the refresh height is in the future, the current code will
loop till the actual height reaches this. Fix it by bailing out
if we receive only three hashes, which is what we set in the
call parameters.

src/wallet/wallet2.cpp

@@ -1601,7 +1601,7 @@ void wallet2::fast_refresh(uint64_t stop_height, uint64_t &blocks_start_height,
   while(m_run.load(std::memory_order_relaxed) && current_index < stop_height)
   {
     pull_hashes(0, blocks_start_height, short_chain_history, hashes);
-    if (hashes.size() < 3)
+    if (hashes.size() <= 3)
       return;
     if (hashes.size() + current_index < stop_height) {
       std::list<crypto::hash>::iterator right;