Used to fault trying to access l->list->next
when l->list == nil after MCentral_AllocList.
Now prints

runtime: out of memory: no room in arena for 65536-byte allocation (536870912 in use)
throw: out of memory

followed by stack trace.

Fixes #1650.

R=r, dfc
CC=golang-dev
https://golang.org/cl/4446062