Clear last_sigs when returning early on abstract

[Morriar]

Before this fix, when encountering a signature with abstract we were not clearing the list, meaning that subsequent signatures were not translated:

sig { abstract.void }
def foo; end

sig { void } # list still contains the abstract one and will return early
def bar; end

After this change, the second signature is properly translated:

sig { abstract.void }
def foo; end

#: -> void
def bar; end

[jesse-shopify]

I like the change from instance variable to local variable for this. This is easier to reason about.