wallet: only use unspent coins when building batch transactions

This will prevent the "Could not resolve preferred inputs" error
that is thrown when a user sends a single reveal followed by a
revealAll

lib/wallet/wallet.js

@@ -2033,7 +2033,7 @@ class Wallet extends EventEmitter {
         continue;
 
       const {hash, index} = prevout;
-      const coin = await this.getCoin(hash, index);
+      const coin = await this.getUnspentCoin(hash, index);
 
       if (!coin)
         continue;
@@ -2300,7 +2300,7 @@ class Wallet extends EventEmitter {
       if (prevout.equals(ns.owner))
         continue;
 
-      const coin = await this.getCoin(hash, index);
+      const coin = await this.getUnspentCoin(hash, index);
 
       if (!coin)
         continue;
@@ -4296,6 +4296,22 @@ class Wallet extends EventEmitter {
     return this.txdb.getCoin(hash, index);
   }
 
+  /**
+   * Get an unspent coin from the wallet.
+   * @param {Hash} hash
+   * @param {Number} index
+   * @returns {Promise} - Returns {@link Coin}.
+   */
+
+  async getUnspentCoin(hash, index) {
+    const credit = await this.txdb.getCredit(hash, index);
+
+    if (credit.spent)
+      return null;
+
+    return credit.coin;
+  }
+
   /**
    * Get a transaction from the wallet.
    * @param {Hash} hash