Fix invalid iterator bump in ValueSplittingHeaderListTest

This was found with _LIBCPP_DEBUG=1. ValueSplittingHeaderListTest tests 6
iterator values, the last of which is headers.end(). That means, at the last
iterator, it cannot run it++.

PiperOrigin-RevId: 279095545
Change-Id: Id59dcb22faee4da4fdd8a4fac080418aa53e8caf

quic/core/qpack/value_splitting_header_list_test.cc

diff --git a/quic/core/qpack/value_splitting_header_list_test.cc b/quic/core/qpack/value_splitting_header_list_test.cc
index 45963bc..bab5238 100644
--- a/quic/core/qpack/value_splitting_header_list_test.cc
+++ b/quic/core/qpack/value_splitting_header_list_test.cc
@@ -60,10 +60,16 @@
         EXPECT_FALSE(it1 == it2);
         EXPECT_TRUE(it1 != it2);
       }
-      ++it2;
+      if (j < kEnd - 1) {
+        ASSERT_NE(it2, headers.end());
+        ++it2;
+      }
     }
 
-    ++it1;
+    if (i < kEnd - 1) {
+      ASSERT_NE(it1, headers.end());
+      ++it1;
+    }
   }
 }