Crash in semweb RDF library

Forked issue from SWI-Prolog/swipl-devel#722. As that one, the problem is supposed to reproduce running the tests highly parallel (-j 12) on a 2-4 CPUs in a loop.

7/69 Test SWI-Prolog/swipl-devel#58: semweb:subprop ...................***Exception: Child aborted  3.64 sec
Seed: 367: ................................................[Thread 4 (__rdf_GC) at Fri Dec  4 10:57:11 2020] ../packages/semweb/rdf_db.c:4403: rdf_gc_info: Assertion failed: (int64_t)garbage >= 0
C-stack trace labeled "assert_fail":
.  [0] save_backtrace() at /home/koom/repos/swipl-devel/build/../src/os/pl-cstack.c:332 [0x7ff3e6e81468]
.  [1] __assert_fail() at /home/koom/repos/swipl-devel/build/../src/pl-assert.c:103 [0x7ff3e6e3bab7]
.  [2] rdf_gc_info() at /home/koom/repos/swipl-devel/build/../packages/semweb/rdf_db.c:4416 [0x7ff3e4aa536a]
.  [3] PL_next_solution() at /home/koom/repos/swipl-devel/build/../src/pl-vmi.c:4177 [0x7ff3e6da42fb]
.  [4] callProlog() at /home/koom/repos/swipl-devel/build/../src/pl-pro.c:394 [0x7ff3e6def89f]
  [5] start_thread() at /home/koom/repos/swipl-devel/build/../src/pl-thread.c:1802 [0x7ff3e6e21859]
.  [6] start_thread() at /build/glibc-S7xCS9/glibc-2.27/nptl/pthread_create.c:463 [0x7ff3e65506db]
......  [7] clone() at /build/glibc-S7xCS9/glibc-2.27/misc/../sysdeps/unix/sysv/linux/x86_64/clone.S:97 [0x7ff3e6a8d71f]

SWI-Prolog / packages-semweb

Crash in semweb RDF library #101