Fixed inconsistencies generated by _md_shrink. (#1229)

This is causing assertion failures in ASSERT_CONSISTENT if an entry used
to have a non null identity. Since the new entries will be concentrated
at the beginning of the array, we shall "erase" the old entries that are
at the end of the array and that won't be used anymore.

This is enough to fix the core dump issues found in the testsuite of
httpie in Fedora Rawhide (see
#1195 (comment))

Author: Romain-Geissler-1A

CHANGES/1229.bugfix.rst

@@ -0,0 +1 @@
+Fixed inconsistencies generated by the C implementation of ``_md_shrink()`` which might later lead to assertion failures and crash -- by :user:`Romain-Geissler-1A`.

multidict/_multilib/hashtable.h

@@ -261,6 +261,7 @@ _md_shrink(MultiDictObject *md, bool update)
     keys->nentries = newnentries;
     keys->usable += nentries - newnentries;
     memset(&keys->indices[0], 0xff, ((size_t)1 << keys->log2_index_bytes));
+    memset(new_ep, 0, sizeof(entry_t) * (size_t)(nentries - newnentries));
     if (htkeys_build_indices(keys, entries, newnentries, update) < 0) {
         return -1;
     }