folly/test: correct an erroneous test for failed mmap

Summary:
* folly/test/AtomicHashArrayTest.cpp (MmapAllocator):
Upon failure, mmap returns MAP_FAILED, not NULL.

Test Plan:
fbconfig -r folly/test:atomic_hash_array_test && fbmake runtests

Reviewed By: mwang@fb.com

FB internal diff: D1481080

Tasks: 4846893

folly/test/AtomicHashArrayTest.cpp

@@ -81,7 +81,7 @@ class MmapAllocator {
   T *allocate(size_t n) {
     void *p = mmap(nullptr, n * sizeof(T), PROT_READ | PROT_WRITE,
         MAP_SHARED | MAP_ANONYMOUS, -1, 0);
-    if (!p) throw std::bad_alloc();
+    if (p == MAP_FAILED) throw std::bad_alloc();
     return (T *)p;
   }