Avoid pointer underflow in CopyCharsUnsigned.

test/cctest/test-api.cc

Index: test/cctest/test-api.cc
diff --git a/test/cctest/test-api.cc b/test/cctest/test-api.cc
index 226d01db62955e0b76d91c14bb89b9fe245a8f30..04664ce2c8e34ee9f346ce250546140a26cf1ed6 100644
--- a/test/cctest/test-api.cc
+++ b/test/cctest/test-api.cc
@@ -6203,6 +6203,10 @@ THREADED_TEST(StringWrite) {
   CHECK_EQ(0, strcmp("abc", buf));
   CHECK_EQ(0, buf[3]);
   CHECK_EQ(0, strcmp("def", buf + 4));
+
+  CHECK_EQ(0, str->WriteAscii(NULL, 0, 0, String::NO_NULL_TERMINATION));
+  CHECK_EQ(0, str->WriteUtf8(NULL, 0, 0, String::NO_NULL_TERMINATION));
+  CHECK_EQ(0, str->Write(NULL, 0, 0, String::NO_NULL_TERMINATION));
 }
 
 
@@ -18127,4 +18131,5 @@ class ThreadInterruptTest {
 THREADED_TEST(SemaphoreInterruption) {
   ThreadInterruptTest().RunTest();
 }
+
 #endif  // WIN32