getApproved require the given token ID exist (#1256)

* getApproved require the given token ID exist

* missing commit from merge

* clarify the test descriptions

contracts/token/ERC721/ERC721Basic.sol

@@ -93,10 +93,12 @@ contract ERC721Basic is ERC165, IERC721Basic {
 
   /**
    * @dev Gets the approved address for a token ID, or zero if no address set
+   * Reverts if the token ID does not exist.
    * @param _tokenId uint256 ID of the token to query the approval of
    * @return address currently approved for the given token ID
    */
   function getApproved(uint256 _tokenId) public view returns (address) {
+    require(_exists(_tokenId));
     return tokenApprovals_[_tokenId];
   }
 

test/token/ERC721/ERC721MintBurn.behavior.js

@@ -84,15 +84,17 @@ function shouldBehaveLikeMintAndBurnERC721 (accounts) {
         });
       });
 
-      describe('when there is a previous approval', function () {
+      describe('when there is a previous approval burned', function () {
         beforeEach(async function () {
           await this.token.approve(accounts[1], tokenId, { from: sender });
           const result = await this.token.burn(tokenId, { from: sender });
           logs = result.logs;
         });
 
-        it('clears the approval', async function () {
-          (await this.token.getApproved(tokenId)).should.be.equal(ZERO_ADDRESS);
+        context('getApproved', function () {
+          it('reverts', async function () {
+            await assertRevert(this.token.getApproved(tokenId));
+          });
         });
       });