check for overflow when doing pointer arithmetic

tests/compile-fail/ptr_offset_overflow.rs

@@ -0,0 +1,8 @@
+//error-pattern: overflowing math on a pointer
+fn main() {
+    let v = [1i8, 2];
+    let x = &v[1] as *const i8;
+    // One of them is guaranteed to overflow
+    let _ = unsafe { x.offset(isize::max_value()) };
+    let _ = unsafe { x.offset(isize::min_value()) };
+}