read-cache: fix leaking hash context in `do_write_index()`

When writing an index with the EOIE extension we allocate a separate
hash context. We never free that context though, causing a memory leak.
Plug it.

This leak is exposed by t9210, but plugging it alone does not make the
whole test suite pass.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

read-cache.c

@@ -3124,6 +3124,7 @@ out:
 	if (f)
 		free_hashfile(f);
 	strbuf_release(&sb);
+	free(eoie_c);
 	free(ieot);
 	return ret;
 }