clang++ compilation fails on clusterfuzz

!1575 (merged) has introduced a compilation failure on clusterfuzz, which I believe uses stock clang++:

Step #4: [137/1205] Compiling C++ object glib/tests/cxx.p/cxx.cpp.o
Step #4: FAILED: glib/tests/cxx.p/cxx.cpp.o 
Step #4: clang++ -Iglib/tests/cxx.p -Iglib/tests -I../../src/glib/glib/tests -I. -I../../src/glib -Iglib -I../../src/glib/glib -Xclang -fcolor-diagnostics -pipe -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -Wnon-virtual-dtor -O2 -g -O1 -fno-omit-frame-pointer -gline-tables-only -DFUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION -fsanitize=address -fsanitize-address-use-after-scope -fsanitize=fuzzer-no-link -stdlib=libc++ -pthread -MD -MQ glib/tests/cxx.p/cxx.cpp.o -MF glib/tests/cxx.p/cxx.cpp.o.d -o glib/tests/cxx.p/cxx.cpp.o -c ../../src/glib/glib/tests/cxx.cpp
Step #4: ../../src/glib/glib/tests/cxx.cpp:43:3: error: cannot initialize a parameter of type 'MyObject **' with an rvalue of type 'std::remove_reference<decltype((nullptr))>::type *' (aka 'nullptr_t *')
Step #4:   g_atomic_pointer_compare_and_exchange (&obj5, nullptr, obj4);
Step #4:   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

/cc @xclaesse