factor out basic line number parsing routine

c64/basic/basic_parse_line_number.s

@@ -0,0 +1,27 @@
+	
+basic_parse_line_number:	
+	
+	jsr injest_number
+
+	;; Check if the number is a valid line number
+	;; i.e., 16 bits, no exponent, no decimal
+	lda basic_fac1_mantissa+2
+	ora basic_fac1_mantissa+3
+	ora basic_fac1_exponent
+	beq +
+ml_bad_line_number:
+	ldx #10
+	jmp do_basic_error
+*	
+	;; Line number are not allowed to have decimal points in them
+	lda tokenise_work4
+	cmp #$ff
+	bne ml_bad_line_number
+
+	;; Got a valid line number.
+	lda basic_fac1_mantissa+0
+	sta basic_line_number+0
+	lda basic_fac1_mantissa+1
+	sta basic_line_number+1
+	clc
+	rts