[tor-bugs] #22521 [Core Tor/Tor]: use offsetof() instead of rolling our own

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#22521: use offsetof() instead of rolling our own
------------------------------+--------------------------------
     Reporter:  catalyst      |      Owner:  catalyst
         Type:  defect        |     Status:  new
     Priority:  Medium        |  Milestone:  Tor: 0.3.2.x-final
    Component:  Core Tor/Tor  |    Version:
     Severity:  Normal        |   Keywords:
Actual Points:                |  Parent ID:
       Points:                |   Reviewer:
      Sponsor:                |
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 `src/common/util.h` defines a `STRUCT_OFFSET()` macro that effectively
 duplicates `offsetof()`, which has been available in `stddef.h` since C89.
 The non-GCC version of `STRUCT_OFFSET()` does pointer arithmetic on null
 pointers, which is undefined behavior.

 We should use the standard `offsetof()` macro instead, unless there's some
 reason we still need to support platforms that lack it.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/22521>
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