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.  */
value='dc67e9e5b2485e7a2f50b3979bdff968028eec42'/><select name='h' onchange='this.form.submit();'>
<option value='koszko' selected='selected'>koszko</option>
<option value='koszko-scripts'>koszko-scripts</option>
</select> <input type='submit' value='switch'/></form></td></tr>
<tr><td class='sub'>Wojtek's customized Guix</td><td class='sub right'></td></tr></table>
<table class='tabs'><tr><td>
<a href='/guix/about/'>about</a><a href='/guix/'>summary</a><a href='/guix/refs/?id=dc67e9e5b2485e7a2f50b3979bdff968028eec42'>refs</a><a class='active' href='/guix/log/build-aux/git-authenticate.scm'>log</a><a href='/guix/tree/build-aux/git-authenticate.scm?id=dc67e9e5b2485e7a2f50b3979bdff968028eec42'>tree</a><a href='/guix/commit/build-aux/git-authenticate.scm?id=dc67e9e5b2485e7a2f50b3979bdff968028eec42'>commit</a><a href='/guix/diff/build-aux/git-authenticate.scm?id=dc67e9e5b2485e7a2f50b3979bdff968028eec42'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/build-aux/git-authenticate.scm'>
<input type='hidden' name='id' value='dc67e9e5b2485e7a2f50b3979bdff968028eec42'/><select name='qt'>
<option value='grep'>log msg</option>
<option value='author'>author</option>
<option value='committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=dc67e9e5b2485e7a2f50b3979bdff968028eec42'>root</a>/<a href='/guix/log/build-aux?id=dc67e9e5b2485e7a2f50b3979bdff968028eec42'>build-aux</a>/<a href='/guix/log/build-aux/git-authenticate.scm?id=dc67e9e5b2485e7a2f50b3979bdff968028eec42'>git-authenticate.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/build-aux/git-authenticate.scm?id=dc67e9e5b2485e7a2f50b3979bdff968028eec42&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-05-07 12:55:14 +0200'>2020-05-07</span></td><td><a href='/guix/commit/build-aux/git-authenticate.scm?id=8d65a71e5fb21303df8628f89e84d2684738f145'>git-authenticate: Use the 'origin/keyring' branch by default.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-05-04 11:08:42 +0200'>2020-05-04</span></td><td><a href='/guix/commit/build-aux/git-authenticate.scm?id=aea6ab2f4ca060e68f8539cd612b0ce088627557'>git-authenticate: Add missing import.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-05-04 09:56:13 +0200'>2020-05-04</span></td><td><a href='/guix/commit/build-aux/git-authenticate.scm?id=041dc3a9c0694ada41b86115b9774a23c9d50f73'>git-authenticate: Load the keyring from the repository.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-05-04 09:56:13 +0200'>2020-05-04</span></td><td><a href='/guix/commit/build-aux/git-authenticate.scm?id=92db1036b7d3ad12548c81450a31e401b4c4f2b5'>git-authenticate: Load the list of authorized keys from the tree.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-05-04 09:56:13 +0200'>2020-05-04</span></td><td><a href='/guix/commit/build-aux/git-authenticate.scm?id=051a45e642ff21908375bee24d272c536096d026'>git-authenticate: Use (guix openpgp).</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-04-17 23:36:47 +0200'>2020-04-17</span></td><td><a href='/guix/commit/build-aux/git-authenticate.scm?id=aa78c596c9eaae946f779d8fa3c4125d08187648'>gnupg: Accept revoked keys.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>