runtime:greyobject: checkmarks finds unexpected unmarked object obj=0xc20803a2a0, mbits->bits=0x1 *mbits->bitp=0x45

[davecheney]

http://build.golang.org/log/5dac94fe99c2e87003812d3d0b5bae282521ec07

# go run run.go -- fixedbugs/issue9321.go
exit status 1
runtime:greyobject: checkmarks finds unexpected unmarked object obj=0xc20803a2a0, mbits->bits=0x1 *mbits->bitp=0x45
runtime:greyobject Span: obj=0xc20803a2a0 k=0x610401d s.start=0xc20803a000 s.limit=0xc20803bfe0 s.sizeclass=4 s.elemsize=48
 *(obj+0) = 0x0
 *(obj+8) = 0xc208055c80
 *(obj+16) = 0x445ae4
 *(obj+24) = 0x5457a0
fatal error: checkmark found unmarked object

runtime stack:
runtime.gothrow(0x52ab50, 0x1f)
        /usr/local/go/src/runtime/panic.go:507 +0x98 fp=0xc208679c08 sp=0xc208679bf0
runtime.greyobject(0xc20803a2a0, 0xc208679ce0, 0x7f6413786fc0, 0x7f6413786fc0)
        /usr/local/go/src/runtime/mgc.go:438 +0x23b fp=0xc208679c68 sp=0xc208679c08
runtime.scanobject(0xc20868db00, 0x72500, 0x0, 0x7f6413786fc0, 0x7f6413786fc0)
        /usr/local/go/src/runtime/mgc.go:567 +0x395 fp=0xc208679d00 sp=0xc208679c68
runtime.scanblock(0xc20868db00, 0x5a70, 0x7f64137810c0)
        /usr/local/go/src/runtime/mgc.go:629 +0x1e6 fp=0xc208679d40 sp=0xc208679d00
runtime.markroot(0xc208010000, 0x0)
        /usr/local/go/src/runtime/mgc.go:638 +0x5e fp=0xc208679db0 sp=0xc208679d40
runtime.parfordo(0xc208010000)
        /usr/local/go/src/runtime/parfor.go:84 +0x1af fp=0xc208679e20 sp=0xc208679db0
runtime.gc(0x210befddea41f, 0x400000000)
        /usr/local/go/src/runtime/mgc.go:1947 +0x303 fp=0xc208679f70 sp=0xc208679e20
runtime.gc_m(0x210befddea41f, 0xc20868d800)
        /usr/local/go/src/runtime/mgc.go:1608 +0x83 fp=0xc208679f90 sp=0xc208679f70
runtime.gccheckmark_m(0x210befddea41f, 0xc208001200)
        /usr/local/go/src/runtime/mgc.go:1776 +0x6e fp=0xc208679fa8 sp=0xc208679f90
runtime.func·010()
        /usr/local/go/src/runtime/malloc.go:538 +0x36 fp=0xc208679fc0 sp=0xc208679fa8
runtime.systemstack(0xc208012900)
        /usr/local/go/src/runtime/asm_amd64.s:244 +0x71 fp=0xc208679fc8 sp=0xc208679fc0
runtime.mstart()
        /usr/local/go/src/runtime/proc1.go:720 fp=0xc208679fd0 sp=0xc208679fc8

[minux]

Dup of #9309.