Segfault merging client configs

[dbresson]

Nomad version

Occurs in Nomad v0.5.4. Error below is from v0.5.2.

Operating system and Environment details

Ubuntu 14.04

Issue

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x4bc7e6]

goroutine 1 [running]:
panic(0xfc0620, 0xc420012050)
/opt/go/src/runtime/panic.go:500 +0x1a1
github.com/hashicorp/nomad/command/agent.(*ClientConfig).Merge(0xc4200f5a20, 0xc4200f5ad0, 0x1133dcc)
/opt/gopath/src/github.com/hashicorp/nomad/command/agent/config.go:883 +0x816
github.com/hashicorp/nomad/command/agent.(*Config).Merge(0xc4200938c0, 0xc420093a20, 0x0)
/opt/gopath/src/github.com/hashicorp/nomad/command/agent/config.go:600 +0x147b
github.com/hashicorp/nomad/command/agent.LoadConfigDir(0x7ffc39f2cf5a, 0x11, 0x0, 0x0, 0x0)
/opt/gopath/src/github.com/hashicorp/nomad/command/agent/config.go:1166 +0x903
github.com/hashicorp/nomad/command/agent.LoadConfig(0x7ffc39f2cf5a, 0x11, 0x1, 0x1, 0x0)
/opt/gopath/src/github.com/hashicorp/nomad/command/agent/config.go:1085 +0x9b
github.com/hashicorp/nomad/command/agent.(*Command).readConfig(0xc42007f900, 0xc420056e70)
/opt/gopath/src/github.com/hashicorp/nomad/command/agent/command.go:165 +0x12aa
github.com/hashicorp/nomad/command/agent.(*Command).Run(0xc42007f900, 0xc42000c110, 0x1, 0x1, 0x0)
/opt/gopath/src/github.com/hashicorp/nomad/command/agent/command.go:420 +0x16d
github.com/hashicorp/nomad/vendor/github.com/mitchellh/cli.(*CLI).Run(0xc4200ce0c0, 0xc4200ce0c0, 0x19, 0xc4200c3920)
/opt/gopath/src/github.com/hashicorp/nomad/vendor/github.com/mitchellh/cli/cli.go:153 +0x330
main.RunCustom(0xc42000c100, 0x2, 0x2, 0xc4201164e0, 0x0)
/opt/gopath/src/github.com/hashicorp/nomad/main.go:55 +0x7a1
main.Run(0xc42000c100, 0x2, 0x2, 0xc4200001a0)
/opt/gopath/src/github.com/hashicorp/nomad/main.go:20 +0x56
main.main()
/opt/gopath/src/github.com/hashicorp/nomad/main.go:16 +0x66

Reproduction steps

I had 1 config file with just enabled = true for the client block, with another config file that loaded after that with the reserved block inside the client block setting a reserved port range. Adding an empty reserved block to the first config file fixed the issue.

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