Fix compiler warning.

Kevin Gritter reports that his compiler complains about inq and outq
being possibly-uninitialized at the point where they are passed to
shm_mq_attach().  They are initialized by the call to
setup_dynamic_shared_memory, but apparently his compiler is inlining
that function and then having doubts about whether the for loop will
always execute at least once.  Fix by initializing them to NULL.

contrib/test_shm_mq/setup.c

@@ -50,8 +50,8 @@ test_shm_mq_setup(uint64 queue_size, int32 nworkers, dsm_segment **segp,
 {
 	dsm_segment *seg;
 	test_shm_mq_header *hdr;
-	shm_mq	   *outq;
-	shm_mq	   *inq;
+	shm_mq	   *outq = NULL;		/* placate compiler */
+	shm_mq	   *inq = NULL;			/* placate compiler */
 	worker_state	   *wstate;
 
 	/* Set up a dynamic shared memory segment. */