check for empty ctx in a2aclr functions
While creating a sample on apple docker a2aclr_dbopen(NULL) returned NULL because a2aclr_dblmdb.c was used instead of a2aclr_dbmem.c because lmdb libraries where available. Then a segfault occured on a2aclr_hasright because that NULL ctx was passed as input. a2aclr_hasright c.s. should always test for NULL ctx and return an appropriate value. see https://github.com/arpa2/apache-httpd/commit/e2f120e9cee849c713f2f016cf26649dc0591a1c
the apple docker works when a file name is passed, eg. a2aclr_dbopen("\tmp\aclr.db");