XLogInsert() of dangling pointer while logging replica identity

Hi, hackers.

  It seems that heapam.c:3082 calls XLogRegisterData() with an argument
allocated on stack, but following call to XLogInsert() happens after
end of context for that variable.
  Issue spotted by clang's AddressSanitizer. Fix attached.

--
Stas Kelvich
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company