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Without this patch, ./temacs would dump core sometimes on Fedora
25 x86-64.  The problem was that the hybrid allocator assumed that
all pointers into bss_sbrk_buffer are allocated via gmalloc.  This
assumption is not true on Fedora, because the standard memory
allocator calls gdefault_morecore, which means its blocks are
interleaved with our blocks.  Usually the code happened to work,
because our data structures agreed with the glibc data structures,
but this was merely luck due to a shared pedigree, and as glibc
mutates our luck has run out.
* src/gmalloc.c (ALLOCATED_BEFORE_DUMPING) [HYBRID_MALLOC]:
Remove; no longer needed.
(BLOCK): Use unsigned division, as that does the right thing near zero.
(register_heapinfo, __malloc_internal_nolock, __free_internal_nolock)
(_realloc_internal_nolock):
Big blocks now have type -1, not 0, as 0 now means the block is
not ours.
(morecore_nolock): Omit now-unnecessary casts to size_t.
(allocated_via_gmalloc) [HYBRID_MALLOC]: New function.
(hybrid_free, hybrid_realloc) [HYBRID_MALLOC]: Use it, to
avoid calling the wrong free or realloc function in some cases.