tools/libs/guest: assist gcc13's realloc analyzer

gcc13 fails to track the allocated memory in backup_ptes:

xg_offline_page.c: In function 'backup_ptes':
xg_offline_page.c:191:13: error: pointer 'orig' may be used after 'realloc' [-Werror=use-after-free]
  191 |             free(orig);

Assist the analyzer by slightly rearranging the code:
In case realloc succeeds, the previous allocation is either extended
or released internally. In case realloc fails, the previous allocation
is left unchanged. Return an error in this case, the caller will
release the currently allocated memory in its error path.

http://bugzilla.suse.com/show_bug.cgi?id=1210570

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Reviewed-by: Juergen Gross <jgross@suse.com>
Compile-tested-by: Jason Andryuk <jandryuk@gmail.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
master commit: 99a9c3d714
master date: 2023-05-03 15:06:41 +0200

tools/libs/guest/xg_offline_page.c

@@ -181,18 +181,14 @@ static int backup_ptes(xen_pfn_t table_mfn, int offset,
 
     if (backup->max == backup->cur)
     {
-        void *orig = backup->entries;
+        void *entries = realloc(backup->entries, backup->max * 2 *
+                                sizeof(struct pte_backup_entry));
 
-        backup->entries = realloc(
-            orig, backup->max * 2 * sizeof(struct pte_backup_entry));
-
-        if (backup->entries == NULL)
-        {
-            free(orig);
+        if (entries == NULL)
             return -1;
-        }
-        else
-            backup->max *= 2;
+
+        backup->entries = entries;
+        backup->max *= 2;
     }
 
     backup->entries[backup->cur].table_mfn = table_mfn;