further multiplication fixes

c64/basic/math/mul_FAC2_FAC1_8x8.s

@@ -6,21 +6,16 @@
 // Helper routine, based on code by djmips from:
 // - https://codebase64.org/doku.php?id=base:8bit_multiplication_16bit_product_fast_no_tables
 //
-// input: .A, INDEX+3
+// input: .A (and Zero flag set according to .A), INDEX+3
 // output: .X (high byte), .A (low byte), carry always clear
 //
 
 mul_FAC2_FAC1_8x8:
 
+	// Special case (optimization) - handle 0 in INDEX+2
+	beq mul_FAC2_FAC1_8x8_zero
+
 	sta INDEX+2
-
-	// Special case (optimization) - handle 0
-
-	lda INDEX+2
-	beq mul_FAC2_FAC1_8x8_zero
-	lda INDEX+3
-	beq mul_FAC2_FAC1_8x8_zero
-
 	lda #$00
 
 	dec INDEX+3	// decrement because we will be adding with carry set for speed (an extra one)