selinux: clean up error path in policydb_init()

Commit e0ac568d ("selinux: reduce the use of hard-coded hash sizes")
moved symtab initialization out of policydb_init(), but left the cleanup
of symtabs from the error path. This patch fixes the oversight.

Suggested-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Paul Moore <paul@paul-moore.com>