Assertion violation about computed heap block address in GC_next_block

[ivmai]

Source: master (57ccbcc)
Build: https://app.travis-ci.com/github/ivmai/bdwgc/jobs/625772331
Host: Ubuntu/ppc64le
Config: CFLAGS_EXTRA="-O3 -D _FORTIFY_SOURCE=2 -D DONT_PROTECT_PTRFREE -D FORCE_MPROTECT_BEFORE_MADVISE -D GC_UNMAPPED_REGIONS_SOFT_LIMIT=120" CONF_OPTIONS="--enable-gc-assertions --enable-cplusplus --enable-static"

Output (disclaimtest):

Assertion failure: headers.c:378

Asserion violation happens at:
c29365b#diff-95225c42602c1f888b9599f55503cded07e197d7f37596ac84fe622ce85e8373R378

                if (allow_free || !HBLK_IS_FREE(hhdr)) {
                    GC_ASSERT(HBLK_ADDR(bi, j) == ADDR(hhdr -> hb_block));
                    return hhdr -> hb_block;

Relates to #627 (commit c29365b)