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From the upstream bug report:

At line 237 in gai_misc.c:

lastp = requests_tail;
if (requests_tail == NULL)
  requests = requests_tail = newp;
else ...

If requests_tail was NULL then lastp remains NULL. 
Unfortunately if gai_create_helper_thread fails, lastp is de-referenced at line 267 which will cause a SIGSEGV.

This should be reproducible with a custom malloc which fails after the right number of allocations.

Upstream commit:

commit bd3b0fbae33a9a4cc5e2daf049443d5cf03d4251
Author: Andreas Schwab <schwab>
Date:   Mon Nov 5 12:47:30 2018 +0100

    libanl: properly cleanup if first helper thread creation failed (bug 22927)

Verified with modified __gai_create_helper_thread() function which always returns -1 and a test that calls getaddrinfo_a(). In glibc-2.17-269.el7 getaddrinfo_a() doesn't crash but returns EAGAIN.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHSA-2019:2118