Fix jump crashes

Fixes #90 and #91. Testing:

```
$ cat segfault1.xml 
<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE scenario SYSTEM "sipp.dtd">
<scenario>
  <send>
    <![CDATA[
      OPTIONS sip:test@test.test SIP/2.0
    ]]>
  </send>
  <nop><action><jump value="1"/></action></nop>
</scenario>
$ ./sipp -sf segfault1.xml localhost
Resolving remote host 'localhost'... Done.
[snip whitespace]
------------------------------ Scenario Screen -------- [1-9]: Change Screen --
  Call-rate(length)   Port   Total-time  Total-calls  Remote-host
  10.0(0 ms)/1.000s   5060       0.00 s            0  127.0.0.1:5060(UDP)

  0 new calls during 0.000 s period      0 ms scheduler resolution
  0 calls (limit 30)                     Peak was 0 calls, after 0 s
  0 Running, 0 Paused, 0 Woken up
  0 dead call msg (discarded)            0 out-of-call msg (discarded)        
  3 open sockets                        

                                 Messages  Retrans   Timeout   Unexpected-Msg
     OPTIONS ---------->         0         0                            
              [ NOP ]              
------ [+|-|*|/]: Adjust rate ---- [q]: Soft exit ---- [p]: Pause traffic -----

2014-09-25	00:14:38.910401	1411600478.910401: Jump statement at index 1 jumps to itself and causes an infinite loop.
------------------------------ Scenario Screen -------- [1-9]: Change Screen --
  Call-rate(length)   Port   Total-time  Total-calls  Remote-host
  10.0(0 ms)/1.000s   5060       0.10 s            1  127.0.0.1:5060(UDP)

  1 new calls during 0.102 s period      1 ms scheduler resolution
  1 calls (limit 30)                     Peak was 1 calls, after 0 s
  0 Running, 2 Paused, 1 Woken up
  0 dead call msg (discarded)            0 out-of-call msg (discarded)        
  3 open sockets                        

                                 Messages  Retrans   Timeout   Unexpected-Msg
     OPTIONS ---------->         1         0                            
              [ NOP ]              
------------------------------ Test Terminated --------------------------------


----------------------------- Statistics Screen ------- [1-9]: Change Screen --
  Start Time             | 2014-09-25	00:14:38.807961	1411600478.807961         
  Last Reset Time        | 2014-09-25	00:14:38.808714	1411600478.808714         
  Current Time           | 2014-09-25	00:14:38.910521	1411600478.910521         
-------------------------+---------------------------+--------------------------
  Counter Name           | Periodic value            | Cumulative value
-------------------------+---------------------------+--------------------------
  Elapsed Time           | 00:00:00:101000           | 00:00:00:102000          
  Call Rate              |    9.901 cps              |    9.804 cps             
-------------------------+---------------------------+--------------------------
  Incoming call created  |        0                  |        0                 
  OutGoing call created  |        1                  |        1                 
  Total Call created     |                           |        1                 
  Current Call           |        1                  |                          
-------------------------+---------------------------+--------------------------
  Successful call        |        0                  |        0                 
  Failed call            |        0                  |        0                 
-------------------------+---------------------------+--------------------------
  Call Length            | 00:00:00:000000           | 00:00:00:000000          
------------------------------ Test Terminated --------------------------------


2014-09-25	00:14:38.910401	1411600478.910401: Jump statement at index 1 jumps to itself and causes an infinite loop.
```

We could do a bit better on this by detecting it when we parse the scenario, like we do for diiding by 0, but parseAction doesn't have the current index as a parameter.

```
$ cat segfault2.xml 
<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE scenario SYSTEM "sipp.dtd">
<scenario>
  <nop><action><jump value="5"/></action></nop>
</scenario>
$ ./sipp -sf segfault2.xml localhost
2014-09-25	00:14:58.549979	1411600498.549979: Unable to determine creation mode of the tool (server, client)
.
```