refactor: minor.

lib/chain/chain.js

@@ -1009,6 +1009,8 @@ Chain.prototype.isBusy = function isBusy() {
 Chain.prototype.add = co(function* add(block) {
   var unlock = yield this.locker.lock(block);
 
+  assert(!this.currentBlock);
+
   this.currentBlock = block.hash('hex');
 
   try {

lib/mempool/mempool.js

@@ -547,25 +547,23 @@ Mempool.prototype.hasReject = function hasReject(hash) {
 
 Mempool.prototype.addTX = co(function* addTX(tx) {
   var unlock = yield this.locker.lock(tx);
-  var missing;
+
+  assert(!this.currentTX);
 
   this.currentTX = tx.hash('hex');
 
   try {
-    missing = yield this._addTX(tx);
+    return yield this._addTX(tx);
   } catch (err) {
     if (err.type === 'VerifyError') {
       if (!tx.hasWitness() && !err.malleated)
         this.rejects.add(tx.hash());
     }
+    throw err;
+  } finally {
     this.currentTX = null;
     unlock();
-    throw err;
   }
-
-  this.currentTX = null;
-  unlock();
-  return missing;
 });
 
 /**

lib/net/pool.js

@@ -1360,7 +1360,6 @@ Pool.prototype._handleTX = co(function* _handleTX(tx, peer) {
     if (err.type === 'VerifyError') {
       if (err.score !== -1)
         peer.reject(tx, err.code, err.reason, err.score);
-      throw err;
     }
     throw err;
   }