Assertion volation about length-based descriptor in GC_mark_from

[ivmai]

Source: master (4f97540)
Build: https://app.travis-ci.com/github/ivmai/bdwgc/jobs/624709039
Host: Linux/x86_64
Compiler: clang-16
Config: CFLAGS_EXTRA="-m32 -funsigned-char -D _FORTIFY_SOURCE=2 -D GC_GCJ_MARK_DESCR_OFFSET=32 -D NO_LONGLONG64" CONF_OPTIONS="--enable-gc-assertions" GCTEST_WITH_MPROTECT_VDB=true
Output (gctest):
...
Emulating dirty bits with mprotect/signals
Assertion failure: extra/../mark.c:700

        case GC_DS_LENGTH:
          /* Large length.  Process part of the range to avoid pushing  */
          /* too much on the stack.                                     */

          /* Either it is a heap object or a region outside the heap.   */
          GC_ASSERT(descr < GC_greatest_real_heap_addr
                            - GC_least_real_heap_addr
                    || GC_least_real_heap_addr + sizeof(ptr_t)
                        >= ADDR(current_p) + descr
                    || ADDR(current_p) >= GC_greatest_real_heap_addr);

[ivmai]

Reproduced locally, both with soft and mprotect-based vdb modes, w/o incremental mode and in single-threaded case:
gcc -I include -I libatomic_ops/src -D GC_GCJ_SUPPORT -O0 -g -m32 -D GC_ASSERTIONS -D GC_GCJ_MARK_DESCR_OFFSET=32 -D NO_TEST_HANDLE_FORK -D NO_INCREMENTAL -D NO_TYPED_TEST tests/gctest.c extra/gc.c

[ivmai]

Related: #168 #137 #92