Fixes issue on breakpoint resume where console was not open upon pausing...

... but opened while paused. Or any other case where either args or args.maxStringLength is not defined.

related to #272 / #29 
<b>Reproduction:</b>
   
Using master 127955f6005c8cbd1c872d01e69149d54b9863ad

1. With the console closed / minimized, set a breakpoint somewhere in the Sources section of the debugger client.
2. In the debugged application, do something that traverses the breakpoint
--> AUT execution pauses, debugger client shows paused. Console is still minimized.
3. Open the console in the debugger client.
4. Click the button to resume execution.

<b>Result:</b>

Error / crash:
```
/usr/local/lib/node_modules/node-inspector/lib/DebuggerClient.js:89
  args.maxStringLength = 10000;
                       ^
TypeError: Cannot set property 'maxStringLength' of undefined
    at DebuggerClient.request (/usr/local/lib/node_modules/node-inspector/lib/DebuggerClient.js:90:24)
    at Object.DebuggerAgent._sendContinue (/usr/local/lib/node_modules/node-inspector/lib/DebuggerAgent.js:132:26)
  ...
```

So this change should protect against that or any other similar cases. 

:zap: : I'm not sure how / if the test should be updated.