Guard call to weak __asan_region_is_poisoned properly

Summary:
It wasn't guarded at all, which is causing issues for non-sanitizer builds on platforms without shared library support.

Ref: https://github.com/facebook/folly/issues/1507

Reviewed By: meyering

Differential Revision: D25970872

fbshipit-source-id: f710e752fdf8ebc5529314d335f4559e690b787a

folly/memory/SanitizeAddress.cpp

@@ -21,13 +21,15 @@
 extern "C" FOLLY_ATTR_WEAK void* __asan_region_is_poisoned(void*, std::size_t);
 
 namespace folly {
-
 namespace detail {
 
 void* asan_region_is_poisoned_(void* const ptr, std::size_t len) {
+  if (kIsLibrarySanitizeAddress) {
     return __asan_region_is_poisoned(ptr, len);
+  } else {
+    return nullptr;
+  }
 }
 
 } // namespace detail
-
 } // namespace folly