Fix off by one in sourceirc-rcon

Off by one error. When the index l is equal to the length of the array linecount, lines[l] will be out of bounds. The highest valid index will be length - 1.

<Alex> I think rcon to trade has died.
<Alex> hmm
<Alex> Ew, wtf?
<Alex> it broke when I typed 'maps *'
<Alex> L 10/03/2014 - 15:46:25: [SM] Plugin encountered error 15: Array index is out of bounds
<Alex> L 10/03/2014 - 15:46:25: [SM] Displaying call stack trace for plugin "SourceIRC/sourceirc-rcon.smx":
<Alex> L 10/03/2014 - 15:46:25: [SM]   [0]  Line 104, scripting/SourceIRC/sourceirc-rcon.sp::OnSocketReceive()