zasm: allow zasm to omit its 3rd argument

A bug in rdWS made zasm error out when omiting its 3rd argument.

fixes #90

apps/lib/util.asm

@@ -20,10 +20,10 @@ toWS:
 ; Set Z if non-WS found, unset if end-of-string.
 rdWS:
 	ld	a, (hl)
-	call	isWS
-	jr	nz, .ok
 	cp	0x01	; if a is null, carries and unsets z
 	ret	c
+	call	isWS
+	jr	nz, .ok
 	inc	hl
 	jr	rdWS
 .ok:

emul/shell/user.h

@@ -1,4 +1,3 @@
-.equ    SHELL_RAMSTART  0x4100
 .equ    USER_CODE       0x4200  ; in sync with glue.asm
 
 ; *** JUMP TABLE ***

tests/unit/test_lib_util.asm

@@ -0,0 +1,55 @@
+jp	test
+
+.inc "ascii.h"
+.inc "core.asm"
+.equ	STDIO_RAMSTART	RAMSTART
+.inc "stdio.asm"
+.inc "common.asm"
+.inc "lib/ari.asm"
+.inc "lib/fmt.asm"
+.inc "lib/util.asm"
+
+test:
+	ld	sp, 0xffff
+
+	call	testRdWS
+
+	; success
+	xor	a
+	halt
+
+testRdWS:
+	ld	hl, .allGood
+	ld	ix, .testGood
+	call	testList
+	ld	hl, .allBad
+	ld	ix, .testBad
+	jp	testList
+
+.testGood:
+	call	rdWS
+	jp	assertZ
+
+.testBad:
+	call	rdWS
+	jp	assertNZ
+
+; Strings ending with a non-WS, and thus yielding Z
+.g1:
+	.db	" X", 0
+.g2:
+	.db	"X", 0
+
+.allGood:
+	.dw	.g1, .g2, 0
+
+; Strings ending with a WS, and thus yielding NZ
+.b1:
+	.db	0
+.b2:
+	.db	" ", 0
+
+.allBad:
+	.dw	.b1, .b2, 0
+
+RAMSTART: