[C++] -Wnull-pointer-arithmetic warning with dlmalloc.c on clang 6.0, Ubuntu 14.04

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I encountered this failure with -Werror on a nearly fresh Ubuntu 14.04 install using clang-6 from LLVM apt

../src/plasma/thirdparty/dlmalloc.c:3504:68: error: arithmetic on a null pointer treated as a cast from integer to pointer is a GNU extension [-Werror,-Wnull-pointer-arithmetic]
      size_t mfree = m->topsize + (((((size_t)(((void*)((char*)(0) + ((sizeof(size_t))<<1)))) & (((size_t)(2 * sizeof(void *))) - ((size_t)1))) == 0)? 0 : ((((size_t)(2 * sizeof(void *))) - ((size_t)(((void*)((char*)(0) + ((sizeof(size_t))<<1)))) & (((size_t)(2 * sizeof(void *))) - ((size_t)1)))) & (((size_t)(2 * sizeof(void *))) - ((size_t)1))))+(((sizeof(struct malloc_segment)) + ((sizeof(size_t))) + (((size_t)(2 * sizeof(void *))) - ((size_t)1))) & ~(((size_t)(2 * sizeof(void *))) - ((size_t)1)))+(((sizeof(mchunk)) + (((size_t)(2 * sizeof(void *))) - ((size_t)1))) & ~(((size_t)(2 * sizeof(void *))) - ((size_t)1))));

See gist for complete log https://gist.github.com/wesm/61ee3592045bfaf390d5d4557f3c5a22

Reporter: Wes McKinney / @wesm
Assignee: Wes McKinney / @wesm

PRs and other links:

• GitHub Pull Request #2099

_{Note: This issue was originally created as ARROW-2668. Please see the migration documentation for further details.}

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Wes McKinney / @wesm:
It looks like this warning was relatively recently added to LLVM https://reviews.llvm.org/D37042. We are using clang-format-6 in our Travis CI but not using clang itself when compiling which is probably why this issue hasn't come up yet

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Uwe Korn / @xhochy:
Issue resolved by pull request 2099
#2099