make fgfs abort() after a listener with runtime arg == -1 was triggered

make fgfs abort() after a listener with runtime arg == -1 was triggered
and its function called. This is useful for debugging purposes, to get
a core dump which unveils which C++ code was writing to the property.
This isn't any more of a security problem as other Nasal code that
makes fgfs crash, such as:  var x = func call(x); x();  The difference
is that the listener doesn't use up all memory before and is much quicker.