core: Fix nil bookie panic in core.Book(...)

[ukane-philemon]

To reproduce, just call the core.Book method for any market.

I already discarded the original log, but this is from Cryptopower:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x1058897]

goroutine 3038 [running]:
decred.org/dcrdex/client/core.(*bookie).translateBookSide(0x0, {0xc009ffa160, 0x3, 0x7?})
        /home/joe/go/pkg/mod/github.com/ukane-philemon/[email protected]/client/core/bookie.go:589 +0x77
decred.org/dcrdex/client/core.(*Core).Book(0xc0037fe000, {0xc0051de020?, 0x1?}, 0x0?, 0x0?)
        /home/joe/go/pkg/mod/github.com/ukane-philemon/[email protected]/client/core/bookie.go:577 +0x891
github.com/crypto-power/cryptopower/ui/page/dcrdex.(*DEXMarketPage).setServerMarkets.func1()
        /home/joe/git/cryptopower/ui/page/dcrdex/market.go:334 +0x64
created by github.com/crypto-power/cryptopower/ui/page/dcrdex.(*DEXMarketPage).setServerMarkets in goroutine 55
        /home/joe/git/cryptopower/ui/page/dcrdex/market.go:332 +0x489

[buck54321]

This can be done in +3 lines master...buck54321:bookpanic

Same thing, right?

[ukane-philemon]

Yh, but I wanted to get rid of the "long" block of if statement.

[buck54321]

Yh, but I wanted to get rid of the "long" block of if statement.

No thank you.

[ukane-philemon]

Okay, one moment.