"Andrzej Hunt via GitGitGadget" [off-list ref] writes:

 void clear_mailinfo(struct mailinfo *mi)
 {
-	int i;
-
 	strbuf_release(&mi->name);
 	strbuf_release(&mi->email);
 	strbuf_release(&mi->charset);
 	strbuf_release(&mi->inbody_header_accum);
 	free(mi->message_id);
 
-	if (mi->p_hdr_data)
-		for (i = 0; mi->p_hdr_data[i]; i++)
-			strbuf_release(mi->p_hdr_data[i]);
-	free(mi->p_hdr_data);
-	if (mi->s_hdr_data)
-		for (i = 0; mi->s_hdr_data[i]; i++)
-			strbuf_release(mi->s_hdr_data[i]);
-	free(mi->s_hdr_data);

So, the original allows mi->p_hdr_data to be NULL and does not do
this freeing (the same for the .s_hdr_data member).

+	strbuf_list_free(mi->p_hdr_data);
+	strbuf_list_free(mi->s_hdr_data);

Is it safe to feed NULL to the helper?

        void strbuf_list_free(struct strbuf **sbs)
        {
                struct strbuf **s = sbs;

                while (*s) {
                        strbuf_release(*s);
                        free(*s++);
                }
                free(sbs);
        }