json: Fix latent parser aborts at end of input

json-parser.c carefully reports end of input like this:

    token = parser_context_pop_token(ctxt);
    if (token == NULL) {
        parse_error(ctxt, NULL, "premature EOI");
        goto out;
    }

Except parser_context_pop_token() can't return null, it fails its
assertion instead.  Same for parser_context_peek_token().  Broken in
commit 65c0f1e9, and faithfully preserved in commit 95385fe9.
Only a latent bug, because the streamer throws away any input that
could trigger it.

Drop the assertions, so we can fix the streamer in the next commit.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180823164025.12553-45-armbru@redhat.com>

qobject/json-parser.c

@@ -226,14 +226,12 @@ out:
 static JSONToken *parser_context_pop_token(JSONParserContext *ctxt)
 {
     g_free(ctxt->current);
-    assert(!g_queue_is_empty(ctxt->buf));
     ctxt->current = g_queue_pop_head(ctxt->buf);
     return ctxt->current;
 }
 
 static JSONToken *parser_context_peek_token(JSONParserContext *ctxt)
 {
-    assert(!g_queue_is_empty(ctxt->buf));
     return g_queue_peek_head(ctxt->buf);
 }