Fix segfault on verify_dominators error path

	* dominance.c (verify_dominators): Don't segfault on NULL imm_bb.

From-SVN: r236845

gcc/ChangeLog

 2016-05-28  Alan Modra  <amodra@gmail.com>
 
+	* dominance.c (verify_dominators): Don't segfault on NULL imm_bb.
+
+2016-05-28  Alan Modra  <amodra@gmail.com>
+
 	PR rtl-optimization/71275
 	* ira.c (ira): Free dominance info.
 

gcc/dominance.c

@@ -1024,6 +1024,7 @@ verify_dominators (cdi_direction dir)
 	{
 	  error ("dominator of %d status unknown", bb->index);
 	  err = true;
+	  continue;
 	}
 
       basic_block imm_bb_correct = di.get_idom (bb);