Bug: Server panic for POST /ob/ordercompletion

[drwasho]

This was the scenario:

Some extra details:

• The payment coin was TLTC
• The slug used in the payload was incorrect, yet the API returned a 200 OK response
• The order details: https://gist.github.com/drwasho/84d645a78cd3ad2e85dbb91b9fae88ad

goroutine 316060 [running]:
runtime/debug.Stack(0x4a3b57b, 0xc0002be4e0, 0x1)
        /usr/local/go/src/runtime/debug/stack.go:24 +0xa7
runtime/debug.PrintStack()
        /usr/local/go/src/runtime/debug/stack.go:16 +0x22
github.com/OpenBazaar/openbazaar-go/api.(*jsonAPIHandler).ServeHTTP.func1()
        /Users/drwasho/Work/go/src/github.com/OpenBazaar/openbazaar-go/api/jsonapi.go:176 +0x10b
panic(0x53353c0, 0x692bf10)
        /usr/local/go/src/runtime/panic.go:513 +0x1b9
github.com/OpenBazaar/openbazaar-go/core.(*OpenBazaarNode).CompleteOrder(0xc000559c20, 0xc0039a5e90, 0xc000b8ab40, 0xc000ac4500, 0x1, 0x4, 0x4, 0x8)
        /Users/drwasho/Work/go/src/github.com/OpenBazaar/openbazaar-go/core/completion.go:115 +0x1eb5
github.com/OpenBazaar/openbazaar-go/api.(*jsonAPIHandler).POSTOrderComplete(0xc0012f2500, 0x5a0b0e0, 0xc0016482a0, 0xc00230c000)
        /Users/drwasho/Work/go/src/github.com/OpenBazaar/openbazaar-go/api/jsonapi.go:2064 +0x474
github.com/OpenBazaar/openbazaar-go/api.(*jsonAPIHandler).POSTOrderComplete-fm(0x5a0b0e0, 0xc0016482a0, 0xc00230c000)
        /Users/drwasho/Work/go/src/github.com/OpenBazaar/openbazaar-go/api/endpoints.go:55 +0x48
github.com/OpenBazaar/openbazaar-go/api.blockingStartupMiddleware(0xc0012f2500, 0x5a0b0e0, 0xc0016482a0, 0xc00230c000, 0xc003e83a98)
        /Users/drwasho/Work/go/src/github.com/OpenBazaar/openbazaar-go/api/endpoints.go:270 +0x62
github.com/OpenBazaar/openbazaar-go/api.post(0xc0012f2500, 0xc00153cfa0, 0x13, 0x5a0b0e0, 0xc0016482a0, 0xc00230c000)
        /Users/drwasho/Work/go/src/github.com/OpenBazaar/openbazaar-go/api/endpoints.go:55 +0x141e
github.com/OpenBazaar/openbazaar-go/api.(*jsonAPIHandler).ServeHTTP(0xc0012f2500, 0x5a0b0e0, 0xc0016482a0, 0xc00230c000)
        /Users/drwasho/Work/go/src/github.com/OpenBazaar/openbazaar-go/api/jsonapi.go:185 +0x8a3
net/http.(*ServeMux).ServeHTTP(0xc00196e1e0, 0x5a0b0e0, 0xc0016482a0, 0xc00230c000)
        /usr/local/go/src/net/http/server.go:2361 +0x127
net/http.serverHandler.ServeHTTP(0xc0011a3ee0, 0x5a0b0e0, 0xc0016482a0, 0xc00230c000)
        /usr/local/go/src/net/http/server.go:2741 +0xab
net/http.(*conn).serve(0xc001c2a0a0, 0x5a0cbe0, 0xc003556400)
        /usr/local/go/src/net/http/server.go:1847 +0x646
created by net/http.(*Server).Serve
        /usr/local/go/src/net/http/server.go:2851 +0x2f5