Guard against creating an infinite loop of jumps
Wait until after verifying the scenario to run screen_init
Check jump statement to be in 0<=jump_value<=TOTAL_MESSAGES.

0 means before the first, TOTAL_MESSAGES means after the last.

Note that we still only catch the simplest form of recursion. The
following will still crash sipp:

    <?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="2"/></action></nop>
      <nop><action><jump value="1"/></action></nop>
    </scenario>
Make next() return control to the scheduler rather than recursing
Merge remote-tracking branch 'origin/fix_jump_crashes' into fix_jump_crashes
Merge pull request #93 from SIPp/fix_jump_crashes

Fix jump crashes