bugfix: initialize possibleToken fields

[Andrew7234]

The analyzer crashed with

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x64bc9d]

goroutine 127299 [running]:
math/big.(*Int).Add(0x0, 0x1?, 0xc0000ab2f0?)
	/usr/local/go/src/math/big/int.go:116 +0x1d
github.com/oasisprotocol/nexus/analyzer/runtime.extractEvents.func4.3({0xc000644b0c, 0x14, 0x14}, {0xc000644b2c, 0x14, 0x14}, {0xc000644b40?, 0xc000602858?, 0x69fab4?})
	/code/go/analyzer/runtime/extract.go:764 +0x5e5
github.com/oasisprotocol/nexus/analyzer/runtime.VisitEVMEvent(0xc0000a5b30, 0xc000602e08)
	/code/go/analyzer/runtime/visitors.go:215 +0x7b9
github.com/oasisprotocol/nexus/analyzer/runtime.extractEvents.func4(0xc0000a5b30)
	/code/go/analyzer/runtime/extract.go:633 +0x456
github.com/oasisprotocol/nexus/analyzer/runtime.VisitSdkEvent(0x8?, 0xc000603220)
	/code/go/analyzer/runtime/visitors.go:148 +0x531
github.com/oasisprotocol/nexus/analyzer/runtime.VisitSdkEvents({0xc000456e00, 0x2, 0x0?}, 0xc000603218?)
	/code/go/analyzer/runtime/visitors.go:158 +0x68
github.com/oasisprotocol/nexus/analyzer/runtime.extractEvents(0xc00054be48?, 0xc000311900?, {0xc000456e00?, 0x2?, 0x6?})
	/code/go/analyzer/runtime/extract.go:529 +0x135
github.com/oasisprotocol/nexus/analyzer/runtime.ExtractRound({0x0, {0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...}, ...}, ...)
	/code/go/analyzer/runtime/extract.go:472 +0x13e7
github.com/oasisprotocol/nexus/analyzer/runtime.(*processor).ProcessBlock(0xc000526230, {0x18e98c0, 0xc000b0c1e0}, 0xc00048e700?)
	/code/go/analyzer/runtime/runtime.go:116 +0x259
github.com/oasisprotocol/nexus/analyzer/block.(*blockBasedAnalyzer).Start(0xc0005262d0, {0x18e9850, 0xc00030f7c0})
	/code/go/analyzer/block/block.go:240 +0x951
github.com/oasisprotocol/nexus/cmd/analyzer.(*Service).Start.func1({0x18e3318?, 0xc0005262d0?})
	/code/go/cmd/analyzer/analyzer.go:387 +0x71
created by github.com/oasisprotocol/nexus/cmd/analyzer.(*Service).Start

The root cause was that the totalSupplyChange could be nil if a previously processed ERC721Approval or ERC721ApprovalForAll event in the block initialized possibleToken.TotalSupplyChange to nil.

Todo:
Might be better to change possibleToken.TotalSupplyChange to not be a pointer since we default to 0 anyway... but big.Int uses pointer receiver methods.

type EVMPossibleToken struct {
	Mutated           bool
	TotalSupplyChange big.Int
}

Testing... wasn't able to connect to a mainnet sapphire grpc endpoint yet locally (only testnet).

[mitjat]

Thank you for the quick fix, Andy! I have a suggestion for making it more future-proof.