Corrects an error in processing of sigscr.dat scrips actions : this_sig_lr and this_sig_mr will now return value '-1' if there is no signalhead of required type availabel for that signal. This is according to the definition of these functions in MSTS.
git-svn-id: http://svn.uktrainsim.com/svn/openrails/trunk@1603 a9f3cfff-6c41-7446-b0ca-052bd761e6eb