Guard local time posix functions from NaN value of invalid dates.

src/platform-posix.cc

Index: src/platform-posix.cc
===================================================================
--- src/platform-posix.cc	(revision 2600)
+++ src/platform-posix.cc	(working copy)
@@ -86,16 +86,20 @@
 }
 
 
-char* OS::LocalTimezone(double time) {
+const char* OS::LocalTimezone(double time) {
+  ASSERT(!isnan(time));
   time_t tv = static_cast<time_t>(floor(time/msPerSecond));
   struct tm* t = localtime(&tv);
-  return const_cast<char*>(t->tm_zone);
+  if (NULL == t) return "";
+  return t->tm_zone;
 }
 
 
 double OS::DaylightSavingsOffset(double time) {
+  ASSERT(!isnan(time));
   time_t tv = static_cast<time_t>(floor(time/msPerSecond));
   struct tm* t = localtime(&tv);
+  if (NULL == t) return nan_value();
   return t->tm_isdst > 0 ? 3600 * msPerSecond : 0;
 }