Don't crash when a layer doesnt have extent set

Stack trace from crash:
```
/home/ubuntu/node_modules/tessera/node_modules/tilelive-tmsource/index.js:92
            memo[key] = tm.extent[id];
                                 ^
TypeError: Cannot read property '900913' of undefined
    at /home/ubuntu/node_modules/tessera/node_modules/tilelive-tmsource/index.js:92:34
    at Function._.each._.forEach (/home/ubuntu/node_modules/tessera/node_modules/tilelive-tmsource/node_modules/underscore/underscore.js:87:22)
    at _.(anonymous function) [as each] (/home/ubuntu/node_modules/tessera/node_modules/tilelive-tmsource/node_modules/underscore/underscore.js:1178:39)
    at /home/ubuntu/node_modules/tessera/node_modules/tilelive-tmsource/index.js:91:19
    at /home/ubuntu/node_modules/tessera/node_modules/tilelive-tmsource/node_modules/underscore/underscore.js:121:25
    at _.each._.forEach (/home/ubuntu/node_modules/tessera/node_modules/tilelive-tmsource/node_modules/underscore/underscore.js:87:22)
    at Function._.reduce._.foldl._.inject (/home/ubuntu/node_modules/tessera/node_modules/tilelive-tmsource/node_modules/underscore/underscore.js:116:5)
    at _.(anonymous function) [as reduce] (/home/ubuntu/node_modules/tessera/node_modules/tilelive-tmsource/node_modules/underscore/underscore.js:1178:39)
    at /home/ubuntu/node_modules/tessera/node_modules/tilelive-tmsource/index.js:84:36
    at Array.map (native)
```