[PATCH] Fix memory leak of primary_sysid in walreceiver

  Hi, Hackers,

  In WalReceiverMain(), the outer streaming loop calls
  walrcv_identify_system() once per iteration to verify the primary's
  system identifier:

      primary_sysid = walrcv_identify_system(wrconn, &primaryTLI);
      ...
      if (strcmp(primary_sysid, standby_sysid) != 0)
          ereport(ERROR, ...);

  walrcv_identify_system() (libpqrcv_identify_system() in
  libpqwalreceiver.c) returns a pstrdup()'d string, but the caller
  never frees it. Each streaming restart therefore leaks the string.
  The error path is unaffected because the surrounding memory context
  is reset on ERROR.

  The attached patch adds a pfree(primary_sysid) right after the
  comparison.

  This dates back to commit 78c8c814390 ("Refactor libpqwalreceiver",
  2016), so it should be a back-patch candidate as well.

  No new tests are added; the fix only releases resources and does not
  change observable behavior. `make check` and the streaming replication
  TAP test (src/test/recovery/t/001_stream_rep.pl) pass with the patch
  applied.

  Patch attached.

  Regards,
  DaeMyung Kang