This code path is exercised indirectly by some of the DNS stub resolver tests, via their own use of xopen_memstream for constructing strings describing result data. The relative lack of test suite coverage became apparent when these tests starting failing after a printf changes uncovered bug 28949.
0060a6de54 stdio-common: Add tst-memstream-string for open_memstream overflow
stdio-common/Makefile | 3 ++
stdio-common/tst-memstream-string.c | 77 +++++++++++++++++++++++++++++++++++++
2 files changed, 80 insertions(+)