zasm: adjust parseLine success condition

De-couple "success" from "wrote something to (DE)" so that directive
parsing can be considered successful without writing to (DE).

apps/zasm/main.asm

@@ -9,9 +9,9 @@ main:
 	ld	bc, 0		; C is our written bytes counter
 .loop:
 	call	parseLine
-	or	a		; is zero? stop
-	jr	z, .stop
-	add	a, c
+	jr	nz, .stop
+	ld	a, c
+	add	a, ixl
 	ld	c, a
 	call	gotoNextLine
 	jr	nz, .stop	; error? stop
@@ -25,8 +25,9 @@ main:
 #include "directive.asm"
 
 ; Parse line in (HL), write the resulting opcode(s) in (DE) and returns the
-; number of written bytes in A. Advances HL where tokenization stopped and DE
+; number of written bytes in IXL. Advances HL where tokenization stopped and DE
 ; to where we should write the next upcode.
+; Sets Z if parse was successful, unset if there was an error or EOF.
 parseLine:
 	push	bc
 
@@ -51,9 +52,14 @@ parseLine:
 	call	copy
 	pop	hl
 	call	JUMP_ADDDE
+	jr	.success
+.success:
+	ld	ixl, a
+	xor	a		; ensure Z
 	jr	.end
 .error:
-	xor	a
+	xor	ixl
+	call	JUMP_UNSETZ
 .end:
 	pop	bc
 	ret