This function accepts a span (of type CDM_Span) and returns the end of
this span. Each span object is a set of exactly two integers, named as
"start" and "end".
Return Value:
This function returns the end value of the provided span object. If
the provided object is not a valid span object or some other error
occurs, tip_GetEnd will return -1 and an error message describing the
error will be placed at the current Tcl interpreter.