qjson: Don't crash when input exceeds nesting limit

We limit nesting depth and input size to defend against input
triggering excessive heap or stack memory use (commit 29c75dd
json-streamer: limit the maximum recursion depth and maximum token
count).  However, when the nesting limit is exceeded,
parser_context_peek_token()'s assertion fails.

Broken in commit 65c0f1e "json-parser: don't replicate tokens at each
level of recursion".

To reproduce stuff 1025 open braces or brackets into QMP.

Fix by taking the error exit instead of the normal one.

Reported-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1448486613-17634-3-git-send-email-armbru@redhat.com>
