scanf: a sign and/or 0x prefix with no following digits should be a matching failure.

This applies to + or - signs for 'd' or 'i' conversions, and to '0x' or '0X' prefixes for 'i' and 'x' conversions. (The new support for signs on u/o/x conversions did not have the problem.)

These cases were treated as if they successfully matched a number, but now they are correctly treated as a failure, causing no assignment to be done and causing the function to return without processing any further directives.

(Incidentally, the handling of '0x' without following digits is something that differs in modern C libraries: macOS libc, glibc, and musl are all different. Our new behavior should match that of musl, which I believe is what is correct under the C standards.)

stdio.asm

@@ -4787,7 +4787,6 @@ ef1	tax		{...back to skipping whitespace}
 	bne	sg2
 	inc	minus
 sg1	jsl	~getchar
-	inc	read
 sg2	ldx	based	if base 8, 16 are allowed then
 	beq	lb2
 	cmp	#'0'	  if the digit is '0' then
@@ -4805,6 +4804,7 @@ lb1a	jsl	~getchar
 	cmp	#'x'
 	bne	lb2
 lb1b	asl	base	      use base 16
+	stz	read	      '0x' alone should not match
 	dec	~scanWidth	      get the next character
 	jeq	lb4a
 	bpl	lb1c
@@ -4903,7 +4903,7 @@ val	ds	8	value
 base	dc	i2'10'	number base
 based	ds	2	based conversion?
 minus	ds	2	is the value negative?
-read	ds	2	# chars read
+read	ds	2	# of digits read
 	end
 
 ****************************************************************
@@ -5228,7 +5228,8 @@ hx1	jsl	~getchar
 	beq	hx1a
 	cmp	#'X'
 	bne	hx2
-hx1a	dec	~scanWidth	    accept the character
+hx1a	stz	read	    ('0x' alone should not match)
+	dec	~scanWidth	    accept the character
 	jeq	lb4a
 	bpl	hx3
 	stz	~scanWidth
@@ -5376,7 +5377,7 @@ val	ds	8	value
 base	dc	i2'10'	number base
 based	ds	2	based conversion?
 minus	ds	2	is there a minus sign?
-read	ds	2	# chars read
+read	ds	2	# of digits read
 	end
 
 ****************************************************************