engine: Fix TestGetOrderbookAmountByNominal panic

[thrasher-]

Thanks to @cranktakular for reporting this:

--- FAIL: TestGetOrderbookAmountByNominal (0.00s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered, repanicked]
[signal 0xc0000005 code=0x0 addr=0x18 pc=0x141c5ce33]

goroutine 3039 [running]:
testing.tRunner.func1.2({0x142198ca0, 0x141f5f490})
        C:/Program Files/Go/src/testing/testing.go:1872 +0x3fc
testing.tRunner.func1()
        C:/Program Files/Go/src/testing/testing.go:1875 +0x683
panic({0x142198ca0?, 0x141f5f490?})
        C:/Program Files/Go/src/runtime/panic.go:783 +0x132
github.com/thrasher-corp/gocryptotrader/engine.TestGetOrderbookAmountByNominal(0xc0005cfdc0)
        C:/Users/emp/Documents/dev/gocryptotrader/engine/rpcserver_test.go:3189 +0xdd3
testing.tRunner(0xc0005cfdc0, 0x142512840)
        C:/Program Files/Go/src/testing/testing.go:1934 +0x1c4
created by testing.(*T).Run in goroutine 1
        C:/Program Files/Go/src/testing/testing.go:1997 +0x9dd
FAIL    github.com/thrasher-corp/gocryptotrader/engine  74.512s
FAIL