oslib-posix: fix memory leak in touch_all_pages

touch_all_pages() can return early, before creating threads.  In this case,
however, it leaks the MemsetContext that it has allocated at the
beginning of the function.

Reported by Coverity as CID 1534922.

Fixes: 04accf43df8 ("oslib-posix: initialize backend memory objects in parallel", 2024-02-06)
Reviewed-by: Mark Kanda <mark.kanda@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

util/oslib-posix.c

@@ -467,11 +467,13 @@ static int touch_all_pages(char *area, size_t hpagesize, size_t numpages,
          * preallocating synchronously.
          */
         if (context->num_threads == 1 && !async) {
+            ret = 0;
             if (qemu_madvise(area, hpagesize * numpages,
                              QEMU_MADV_POPULATE_WRITE)) {
-                return -errno;
+                ret = -errno;
             }
-            return 0;
+            g_free(context);
+            return ret;
         }
         touch_fn = do_madv_populate_write_pages;
     } else {