runtime: Better test tracebackability of jmpdefer when running a nil defer.
runtime: Better test tracebackability of jmpdefer when running a nil defer.

R=bradfitz, dvyukov
CC=golang-dev
https://codereview.appspot.com/12536046

pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,tv42/old-go,tv42/old-go,tv42/old-go,apparentlymart/golang,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang