sdk/queue: move lock before checking queue length

[jasonodonnell]

A panic can occur when head.Pop is called if the length of the queue has changed since checking if it was empty. This is because head.Pop calls swap under the hood. Moving the lock prior to checking should prevent a panic from happening.

Panic as seen in OpenLDAP secret engine:

Nov 01 13:32:38  vault[3385105]: panic: runtime error: index out of range [0] with length 0
4Nov 01 13:32:38  vault[3385105]: goroutine 10378 [running]:
5Nov 01 13:32:38  vault[3385105]: github.com/hashicorp/vault/sdk/queue.queue.Swap(...)
6Nov 01 13:32:38  vault[3385105]:         /gopath/src/github.com/hashicorp/vault/sdk/queue/priority_queue.go:167
7Nov 01 13:32:38  vault[3385105]: container/heap.Pop(0x62f5198, 0xc010b59c80, 0x8b16dc0, 0xc001a30d50)
8Nov 01 13:32:38  vault[3385105]:         /goroot/src/container/heap/heap.go:62 +0x64
9Nov 01 13:32:38  vault[3385105]: github.com/hashicorp/vault/sdk/queue.(*PriorityQueue).Pop(0xc010b59c80, 0x0, 0x0, 0x0)
10Nov 01 13:32:38  vault[3385105]:         /gopath/src/github.com/hashicorp/vault/sdk/queue/priority_queue.go:95 +0xa7
11Nov 01 13:32:38  vault[3385105]: github.com/hashicorp/vault-plugin-secrets-openldap.(*backend).popFromRotationQueue(0xc01bf67e00, 0x0, 0x0, 0x0)
12Nov 01 13:32:38  vault[3385105]:         /gopath/pkg/mod/github.com/hashicorp/vault-plugin-secrets-openldap@v0.5.2/rotation.go:545 +0x85
13Nov 01 13:32:38  vault[3385105]: github.com/hashicorp/vault-plugin-secrets-openldap.(*backend).rotateCredential(0xc01bf67e00, 0x62dd268, 0xc010b59cc0, 0x62dee68, 0xc002b7d8c0, 0x0)
14Nov 01 13:32:38  vault[3385105]:         /gopath/pkg/mod/github.com/hashicorp/vault-plugin-secrets-openldap@v0.5.2/rotation.go:155 +0x8d
15Nov 01 13:32:38  vault[3385105]: github.com/hashicorp/vault-plugin-secrets-openldap.(*backend).rotateCredentials(...)
16Nov 01 13:32:38  vault[3385105]:         /gopath/pkg/mod/github.com/hashicorp/vault-plugin-secrets-openldap@v0.5.2/rotation.go:144
17Nov 01 13:32:38  vault[3385105]: github.com/hashicorp/vault-plugin-secrets-openldap.(*backend).runTicker(0xc01bf67e00, 0x62dd268, 0xc010b59cc0, 0x62dee68, 0xc002b7d8c0)
18Nov 01 13:32:38  vault[3385105]:         /gopath/pkg/mod/github.com/hashicorp/vault-plugin-secrets-openldap@v0.5.2/rotation.go:112 +0x116
19Nov 01 13:32:38  vault[3385105]: created by github.com/hashicorp/vault-plugin-secrets-openldap.(*backend).initQueue
20Nov 01 13:32:38  vault[3385105]:         /gopath/pkg/mod/github.com/hashicorp/vault-plugin-secrets-openldap@v0.5.2/rotation.go:449 +0x165
21Nov 01 13:32:44  vault[3391153]: Fortanix C_GetFunctionList -> 0