Fix concurrent access to blockCtx in debug_traceBlock

[hkalina]

Fix nil panics caused by concurrent access to blockCtx (and GetHashFn function in it).

This is related to following panics:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0xd8b626]

goroutine 212151913 [running]:
github.com/Fantom-foundation/go-opera/evmcore.GetHashFn.func1(0x1adf0d9)
	/go/go-opera/evmcore/evm.go:78 +0x126
github.com/ethereum/go-ethereum/core/vm.opBlockhash(0x0, 0x0, 0x0)
	/go/pkg/mod/github.com/!fantom-foundation/go-ethereum@v1.10.8-ftm-rc4/core/vm/instructions.go:448 +0xa6
github.com/ethereum/go-ethereum/core/vm.(*EVMInterpreter).Run(0xc01201e000, 0xc051d3fa40, {0xc04a20a3c0, 0x44, 0x44}, 0x0)
	/go/pkg/mod/github.com/!fantom-foundation/go-ethereum@v1.10.8-ftm-rc4/core/vm/interpreter.go:263 +0xac4
github.com/ethereum/go-ethereum/core/vm.(*EVM).Call(0xc08885ea80, {0x157c340, 0xc0aed6a930}, {0x24, 0x1, 0x7c, 0x94, 0x10, 0x8c, 0xef, ...}, ...)
	/go/pkg/mod/github.com/!fantom-foundation/go-ethereum@v1.10.8-ftm-rc4/core/vm/evm.go:237 +0xceb
github.com/Fantom-foundation/go-opera/evmcore.(*StateTransition).TransitionDb(0xc0753cb730)
	/go/go-opera/evmcore/state_transition.go:292 +0x58e
github.com/Fantom-foundation/go-opera/evmcore.ApplyMessage(0xc07401b440, {0x15b3da0, 0xc07401b440}, 0x0)
	/go/go-opera/evmcore/state_transition.go:173 +0x32
github.com/Fantom-foundation/go-opera/ethapi.(*PublicDebugAPI).traceTx(0xc03c3fa010, {0x159a998, 0xc0a0c99680}, {0x15b3da0, 0xc07401b440}, 0xc00c529df8, {0x1429ba0, 0x1429ba8, 0xc06d497e00, {0x0, ...}, ...}, ...)
	/go/go-opera/ethapi/api.go:2070 +0x5a9
github.com/Fantom-foundation/go-opera/ethapi.(*PublicDebugAPI).traceBlock.func1()
	/go/go-opera/ethapi/api.go:2196 +0x406
created by github.com/Fantom-foundation/go-opera/ethapi.(*PublicDebugAPI).traceBlock
	/go/go-opera/ethapi/api.go:2186 +0x2c5

[hkalina]

For reference, there is also similar issue and fix in go-ethereum from January: ethereum/go-ethereum#24286