pthread_frame: minor simplification to error handling

pthread_frame: minor simplification to error handling

Get rid of the "ret" variable, and always use err. Report the packet as
consumed if err is unset. This should be equivalent to the old code,
which obviously required err=0 for p->result>=0 (and otherwise,
p->result must have had the value err was last set to). The code block
added by commit 32a5b631267 is also not needed anymore, because the new
code strictly returns err if it's >=0.

Reviewed-by: "Ronald S. Bultje" <rsbultje@gmail.com>