[openacc, parloops] Fix SIGSEGV in oacc_entry_exit_ok_1

[openacc, parloops] Fix SIGSEGV in oacc_entry_exit_ok_1

When compiling the test-case with r268755, we run into a SIGSEGV in
oacc_entry_exit_ok_1 when trying to dereference a NULL red:
...
                      struct reduction_info *red;
                      red = reduction_phi (reduction_list, use_stmt);
                      tree val = PHI_RESULT (red->keep_res);
...

Fix this by handling ref == NULL.

Bootstrapped and reg-tested on x86_64.
Build and reg-tested on x86_64 with nvptx accelerator.

2019-06-16  Tom de Vries  <tdevries@suse.de>

	PR tree-optimization/89376
	* tree-parloops.c (oacc_entry_exit_ok_1): Handle red == NULL.

	* testsuite/libgomp.oacc-c-c++-common/pr89376.c: New test.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@272338 138bc75d-0d04-0410-961f-82ee72b054a4