Gyruss: Routine at 9C92

9C92: Handle relative jump

Used by the routines at L9BF8 and L9C46.
update_counter_and_jump	9C92	LD E,A	Save tune byte
	9C93	LD A,(IX+$09)	Get countdown
	9C96	AND A	Was it zero?
	9C97	JR Z,relative_jump	Then set it and make relative jump
	9C99	DEC (IX+$09)	Else decrement countdown
	9C9C	JR NZ,relative_jump_0	If not zero then make relative jump
	9C9E	INC HL	Else next tune address
	9C9F	RET