gc: fix crash on complicated arg to make slice.
gc: fix crash on complicated arg to make slice.

Fixes issue 615.

R=ken2
CC=golang-dev
http://codereview.appspot.com/255043

apparentlymart/golang,apparentlymart/golang,tv42/old-go,gnanderson/go,tianon/debian-golang,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,h8liu/golang,tianon/debian-golang,tianon/debian-golang,nightlyone/golang,tianon/debian-golang,nightlyone/golang,gnanderson/go,dsriram/golang,ssrl/go,h8liu/golang,bdwalton/debian-golang,tianon/debian-golang,nightlyone/golang,gnanderson/go,ssrl/go,h8liu/golang,gnanderson/go,tianon/debian-golang,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,pombredanne/go-deleteme,nightlyone/golang,gnanderson/go,pombredanne/go-deleteme,bdwalton/debian-golang,h8liu/golang,bdwalton/debian-golang,bdwalton/debian-golang,apparentlymart/golang,dsriram/golang,h8liu/golang,dsriram/golang,ssrl/go,pombredanne/go-deleteme,ssrl/go,ssrl/go,tianon/debian-golang,bdwalton/debian-golang,tv42/old-go,h8liu/golang,dsriram/golang,nightlyone/golang,pombredanne/go-deleteme,h8liu/golang,tv42/old-go,ssrl/go,dsriram/golang,bdwalton/debian-golang,dsriram/golang,tv42/old-go,dsriram/golang,tv42/old-go,tianon/debian-golang,gnanderson/go,bdwalton/debian-golang,tv42/old-go,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,nightlyone/golang,bdwalton/debian-golang,ssrl/go,nightlyone/golang