fix vm.due_insn_count subtract overflow when enable_instruction_meter=false

[hycinth22]

I found a corner case.

if config.enable_instruction_meter=false & jit is off,
vm.due_insn_count subtract overflow encoutered at

sbpf/src/interpreter.rs

Line 590 in 946f7d9

self.vm.due_insn_count = self.vm.previous_instruction_meter - self.vm.due_insn_count;

that's because we set previous_instruction_meter to 0 when config.enable_instruction_meter is set to false

sbpf/src/vm.rs

Lines 347 to 352 in 946f7d9

	let initial_insn_count = if config.enable_instruction_meter {
	self.context_object_pointer.get_remaining()
	} else {
	0
	};
	self.previous_instruction_meter = initial_insn_count;

I confirmed that it exists in interpreter mode,
but I'm not sure about the behavior in JIT mode (request for review)