Check overflow in Loop_Integer

or this code will cause an infinite loop:

>> a: 9223372036854775807
== 9223372036854775807

>> for b 0 a to integer! a / 2 [print b]

With this commit:
>> for b 0 a to integer! a / 2 [print b]
0
4611686018427387904
** Math error: math or number overflow
** Where: for
** Near: for b 0 a to integer! a / 2 [print b]

src/core/n-loop.c

@@ -29,6 +29,7 @@
 ***********************************************************************/
 
 #include "sys-core.h"
+#include "sys-int-funcs.h" //REB_I32_ADD_OF
 
 
 /***********************************************************************
@@ -125,12 +126,16 @@
 
 	VAL_SET(var, REB_INTEGER);
 
-	for (; (incr > 0) ? start <= end : start >= end; start += incr) {
+	while ((incr > 0) ? start <= end : start >= end) {
 		VAL_INT64(var) = start;
 		result = Do_Blk(body, 0);
 		if (THROWN(result) && Check_Error(result) >= 0) break;
 		if (!IS_INTEGER(var)) Trap_Type(var);
 		start = VAL_INT64(var);
+
+		if (REB_I64_ADD_OF(start, incr, &start)) {
+			Trap0(RE_OVERFLOW);
+		}
 	}
 }