Cancelling two simultaneous IP allocation requests crashes weave

[bboreham]

Due to dumb coding in alloc.cancelOpsFor()

To reproduce:

$ sudo weave launch --init-peer-count=2 
$ sudo docker run -tdi ubuntu
df7fe26b8d870e7090662b4e36edf988c4d1df0545a6ee1e1bcaf3999ebe2486
$ sudo weave attach df7 &
[1] 32082
$ sudo weave attach df7 &
[2] 32090
$ sudo docker kill df7
[attach commands fail and exit]
$ sudo docker logs weave
...
panic: runtime error: slice bounds out of range

goroutine 16 [running]:
github.com/weaveworks/weave/ipam.(*Allocator).cancelOpsFor(0xc2090d4300, 0xc2090d4360, 0xc2092e3100, 0x40, 0xd261a0)
    /home/bryan/golang/src/github.com/weaveworks/weave/ipam/allocator.go:133 +0x2fa
github.com/weaveworks/weave/ipam.func·004()
    /home/bryan/golang/src/github.com/weaveworks/weave/ipam/allocator.go:249 +0x1b6
...

(note this was on a dev branch so line numbers may be slightly off, but the bug is on master)