RFI 0.28: Recent update introduced a segfault in store when initializing a non-existent queue

A recent update (r.1578899) introduced a bug into the store in which
initializing a non-existent queue caused a segfualt. The fix is both
obvious and trivial.

JIRA: https://issues.apache.org/jira/browse/QPID-5651

Review Board: https://reviews.apache.org/r/20003/