Fix termination check of x in *

[tompng]

Fix #1080

It is hard to distinguish multiplication from pattern match from tokens.
Token * are both [[3, 0], :on_op, "*", BEG].

# x in *
<<A; x in
A
*

# string * value(missing)
<<A\
A
*

So in this pull request, treat this :on_op, '*', BEG as should_continue == true only when the whole code is syntax valid.

Also fixes x in ** x => * x => ** and def f(*,**)=f *, **