PR middle-end/48591
* omp-low.c (expand_omp_atomic_fetch_op): Return false if decl is
NULL.
(expand_omp_atomic_pipeline): Return false if cmpxchg is NULL.
* gcc.dg/gomp/pr48591.c: New test.
* testsuite/libgomp.c/pr48591.c: New test.
From-SVN: r172379