create error on negative dsb offsets

xa/src/xat.c

@@ -1213,7 +1213,9 @@ fprintf(stderr, "mvn mvp: %i %i %i %i %i\n", t[0], t[i], wide, i, j);
 			 er=E_ILLPOINTER;
 		    else
 */
-                    {
+		    if (j < 0) {
+			er = E_ILLQUANT;
+		    } else {
 			 dsb_len = 0;
 
 			 if(t[i+1]==',') {

xa/tests/dsb/Makefile

@@ -0,0 +1,12 @@
+default: test1 test2 
+
+test1:
+	../../xa -R -LLIB6502 test1.s -o test1.o
+	../hextool -cmp=ok1 < test1.o
+
+test2:
+	# this test must fail.
+	../../xa -R -LLIB6502 test2.s -o test2.o || exit 0 && exit 1
+
+clean:
+	rm -f *.o

xa/tests/dsb/test1.s

@@ -0,0 +1,9 @@
+
+	.text
+
+	.dsb 10, 32
+
+	.dsb 5, 'a'
+
+
+

xa/tests/dsb/test2.s

@@ -0,0 +1,9 @@
+
+	.text
+
+	.dsb -10, 32
+
+	.dsb 5, 'a'
+
+
+