Allow the time remaining parameter to nanosleep to be null

Summary: Pah! This definitely wasn't causing the lock tests to segfault. Nope, not at all <_>.

Reviewed By: yfeldblum

Differential Revision: D3691120

fbshipit-source-id: 6601db637f22c7b0bc326907a2a7976f6df7c159

folly/portability/Time.cpp

@@ -320,8 +320,10 @@ tm* localtime_r(const time_t* t, tm* o) {
 
 int nanosleep(const struct timespec* request, struct timespec* remain) {
   Sleep((DWORD)((request->tv_sec * 1000) + (request->tv_nsec / 1000000)));
-  remain->tv_nsec = 0;
-  remain->tv_sec = 0;
+  if (remain != nullptr) {
+    remain->tv_nsec = 0;
+    remain->tv_sec = 0;
+  }
   return 0;
 }