libs/guest: Fix leak on realloc failure in backup_ptes()

From `man 2 realloc`:

  If realloc() fails, the original block is left untouched; it is not freed or moved.

Found using GCC -fanalyzer:

  |  184 |         backup->entries = realloc(backup->entries,
  |      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  |      |         |               | |
  |      |         |               | (91) when ‘realloc’ fails
  |      |         |               (92) ‘old_ptes.entries’ leaks here; was allocated at (44)
  |      |         (90) ...to here

Signed-off-by: Edwin Török <edwin.torok@cloud.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>

tools/libs/guest/xg_offline_page.c

@@ -181,10 +181,16 @@ static int backup_ptes(xen_pfn_t table_mfn, int offset,
 
     if (backup->max == backup->cur)
     {
-        backup->entries = realloc(backup->entries,
-                            backup->max * 2 * sizeof(struct pte_backup_entry));
+        void *orig = backup->entries;
+
+        backup->entries = realloc(
+            orig, backup->max * 2 * sizeof(struct pte_backup_entry));
+
         if (backup->entries == NULL)
+        {
+            free(orig);
             return -1;
+        }
         else
             backup->max *= 2;
     }