Fix crash due to: <https://issues.guix.gnu.org/issue/39266>.

diff --git a/libguile/struct.c b/libguile/struct.c
index 3dbcc71d4..ddcbe46d2 100644
--- a/libguile/struct.c
+++ b/libguile/struct.c
@@ -139,7 +139,9 @@ set_vtable_access_fields (SCM vtable)
   nfields = len / 2;
 
   bitmask_size = (nfields + 31U) / 32U;
-  unboxed_fields = scm_gc_malloc_pointerless (bitmask_size, "unboxed fields");
+  unboxed_fields =
+    scm_gc_malloc_pointerless (bitmask_size * sizeof (*unboxed_fields),
+                               "unboxed fields");
   memset (unboxed_fields, 0, bitmask_size * sizeof(*unboxed_fields));
 
   /* Update FLAGS according to LAYOUT.  */