Crash on OP_UTXOTOKENCATEGORY

[dagurval]

When executing OP_UTXOTOKENCATEGORY (F4) followed by step back and forward again (so F4 F3 F4) causes a crash.

Example:

% meep debug -t 499ca8e57d0ebd358f159f4ca2140ad1c3efd851bb0cc43af28c13a9d107f7b5
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0xda83178]

goroutine 1 [running]:
github.com/gcash/bchd/txscript.(*UtxoCache).GetEntry(0x0, 0xd67cd33?)
	/Users/dagurval/bchd/txscript/hashcache.go:127 +0x58
github.com/gcash/bchd/txscript.opcodeUtxoTokenCategory(0xd6259e9?, 0xc000499400)
	/Users/dagurval/bchd/txscript/opcode.go:3298 +0xea
github.com/gcash/bchd/txscript.(*Engine).executeOpcode(0xc000499400?, 0xc0003e7560?)
	/Users/dagurval/bchd/txscript/engine.go:240 +0x2a4
github.com/gcash/bchd/txscript.(*Engine).Step(0xc000499400)
	/Users/dagurval/bchd/txscript/engine.go:389 +0x85
main.(*Debug).Execute(0xc0000c69b0, {0xdd351e0?, 0x7ff7b3973801?, 0x1?})
	/Users/dagurval/meep/debug.go:380 +0x2cfe
github.com/jessevdk/go-flags.(*Parser).ParseArgs(0xc0000c53b0, {0xc0000340d0, 0x3, 0x3})
	/Users/dagurval/go/pkg/mod/github.com/jessevdk/go-flags@v1.6.1/parser.go:339 +0xae3
github.com/jessevdk/go-flags.(*Parser).Parse(...)
	/Users/dagurval/go/pkg/mod/github.com/jessevdk/go-flags@v1.6.1/parser.go:195
main.main()
	/Users/dagurval/meep/main.go:33 +0x1a5