xen/rangeset: fix incorrect subtraction

Given the following rangset operation:

{ [0, 1], [4, 5] } - { [3, 4] }

The current rangeset logic will output a rangeset:

{ [0, 2], [5, 5] }

This is incorrect, and also has the undesirable property of being bogus in
a way that the resulting rangeset is expanded.

Fix this by making sure the bounds are correctly checked before modifying
the previous range.

Fixes: 484a058c48 ('Add auto-destructing per-domain rangeset data structure...')
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/common/rangeset.c

@@ -227,7 +227,8 @@ int rangeset_remove_range(
 
         if ( x->s < s )
         {
-            x->e = s - 1;
+            if ( x->e >= s )
+                x->e = s - 1;
             x = next_range(r, x);
         }