vdp: fix 0 checks across various versions/ports

As noted in #994

Kernel/dev/vdp1.s

@@ -473,6 +473,9 @@ clear_lines:
 	    pop de	; E = line, D = count
 	    push de
 	    push hl
+	    xor a
+	    cp d
+	    ret z
 	    ld a,(_int_disabled)
 	    push af
 	    di
@@ -509,6 +512,9 @@ clear_across:
 	    push bc
 	    push de
 	    push hl
+	    xor a
+	    cp c
+	    ret z
 	    ld a,(_int_disabled)
 	    push af
 	    di

Kernel/platform-geneve/vdp.c

@@ -47,7 +47,8 @@ void clear_lines(int8_t y, int8_t ct)
 
     addr = VT_BASE + vt_offset + y * VT_WIDTH;
     v99xx_set_vram_page(addr / V99xx_VRAM_PAGE_SIZE);
-    v99xx_memset_vram(addr, ' ',  ct * VT_WIDTH);
+    if (ct)
+        v99xx_memset_vram(addr, ' ',  ct * VT_WIDTH);
 }
 
 void clear_across(int8_t y, int8_t x, int16_t l)
@@ -56,7 +57,8 @@ void clear_across(int8_t y, int8_t x, int16_t l)
 
     addr = VT_BASE + vt_offset + y * VT_WIDTH + x;
     v99xx_set_vram_page(addr / V99xx_VRAM_PAGE_SIZE);
-    v99xx_memset_vram(addr, ' ', l);
+    if (l)
+        v99xx_memset_vram(addr, ' ', l);
 }
 
 void cursor_off(void)

Kernel/platform-mtx/vdp.s

@@ -133,6 +133,9 @@ _clear_across:
 	     push bc
 	     push de
 	     push hl
+	     xor a
+	     cp c
+	     ret z
 	     call videopos6		; HL is now the offset we need
 	     ld b, c
 clearbyte:
@@ -157,6 +160,9 @@ _clear_lines:
 	     pop de
 	     push de		; E = line, D = count
 	     push hl
+	     xor a
+	     cp d
+	     ret z
 	     ld c,d		; Lines to copy
 	     ld d, #0		; E = y, D = X
 	     ld b, #80

Kernel/platform-rc2014/vdp1.s

@@ -262,6 +262,9 @@ _tms_clear_lines:
 	    push de
 	    push hl
 	    push bc
+	    xor a
+	    cp d
+	    ret z
 	    ld a,(_int_disabled)
 	    push af
 	    di
@@ -295,6 +298,9 @@ _tms_clear_across:
 	    push de
 	    push hl
 	    push af
+	    xor a
+	    cp c
+	    ret z
 	    ld a,(_int_disabled)
 	    push af
 	    di

Kernel/platform-rcbus-8085/vdp1.S

@@ -205,6 +205,9 @@ _clear_lines:
 		ldsi 2
 		lhlx	; Check do we push 2 x 16bit or 2 x 8bit ? FIXME
 		xchg	; E = line, D = count
+	        xra a
+	        cmp d
+                rz
 		lda _int_disabled
 		push psw
 		di
@@ -240,6 +243,8 @@ _clear_across:
 		xchg			; DE = coords
 		ldsi 2
 		ldax d
+	        ora a
+		rz
 		mov c,a			; C = count
 
 		lda _int_disabled