Fix Win32 bots - they crash/timeout on too long thread name.

src/platform-win32.cc

Index: src/platform-win32.cc
===================================================================
--- src/platform-win32.cc	(revision 7294)
+++ src/platform-win32.cc	(working copy)
@@ -754,9 +754,13 @@
 
 
 void OS::StrNCpy(Vector<char> dest, const char* src, size_t n) {
+  // Use _TRUNCATE or strncpy_s crashes (by design) if buffer is too small.
+  size_t buffer_size = static_cast<size_t>(dest.length());
+  if (n + 1 > buffer_size) // count for trailing '\0' 
+    n = _TRUNCATE;
   int result = strncpy_s(dest.start(), dest.length(), src, n);
   USE(result);
-  ASSERT(result == 0);
+  ASSERT(result == 0 || (n == _TRUNCATE && result == STRUNCATE)); 
 }