Fix crash when IntervalMapOverlaps::advanceTo moves past the last overlap.

llvm-svn: 122081
diff --git a/llvm/unittests/ADT/IntervalMapTest.cpp b/llvm/unittests/ADT/IntervalMapTest.cpp
index e0d95a1..eb1f1a4 100644
--- a/llvm/unittests/ADT/IntervalMapTest.cpp
+++ b/llvm/unittests/ADT/IntervalMapTest.cpp
@@ -586,7 +586,11 @@
   ASSERT_TRUE(BA.valid());
   EXPECT_EQ(3u, BA.a().start());
   EXPECT_EQ(4u, BA.b().start());
-  ++BA;
+  // advance past end.
+  BA.advanceTo(6);
+  EXPECT_FALSE(BA.valid());
+  // advance an invalid iterator.
+  BA.advanceTo(7);
   EXPECT_FALSE(BA.valid());
 }