Don't crash if the tm_zone field returned by localtime is NULL

src/base/platform/platform-linux.cc

Index: src/base/platform/platform-linux.cc
diff --git a/src/base/platform/platform-linux.cc b/src/base/platform/platform-linux.cc
index eff5ced3b75f4c6f993b7548b47fbaf5aee506e6..b13a1e85c540eadf54a23735482c67c8b3bd9c0a 100644
--- a/src/base/platform/platform-linux.cc
+++ b/src/base/platform/platform-linux.cc
@@ -109,7 +109,7 @@ const char* OS::LocalTimezone(double time, TimezoneCache* cache) {
   if (std::isnan(time)) return "";
   time_t tv = static_cast<time_t>(std::floor(time/msPerSecond));
   struct tm* t = localtime(&tv);
-  if (NULL == t) return "";
+  if (!t || !t->tm_zone) return "";
   return t->tm_zone;
 #endif
 }