FIX: disallow subtracting date from integer

fixes: Oldes/Rebol-issues#276

Author: Oldes

src/core/t-date.c

@@ -746,6 +746,7 @@
 
 		if (type == REB_DATE) {
 			if (action == A_SUBTRACT) {
+				if(!IS_DATE(val)) Trap_Math_Args(REB_INTEGER, A_SUBTRACT);
 				num = Diff_Date(date, VAL_DATE(arg));
 				goto ret_int;
 			}

src/tests/units/date-test.r3

@@ -130,6 +130,10 @@ Rebol [
 		--assert error? try [load "3-Jan-2010/10:00+2:00:56"]
 		--assert error? try [load "3-Jan-2010/10:00+2:00:56.1234pm"]
 
+	--test-- "issue-276"
+		;@@ https://github.com/Oldes/Rebol-issues/issues/276
+		--assert error? try [0 - 1-jan-0000] ;- no crash!
+
 ===end-group===
 
 ===start-group=== "DATE/TIMEZONE"