revert min_allocator default construction

Author: vinay-deshmukh

libcxx/test/support/min_allocator.h

@@ -212,7 +212,7 @@ class min_pointer<void, ID> {
 
 template <class T, class ID>
 class min_pointer {
-  T* ptr_ {};
+  T* ptr_;
 
   TEST_CONSTEXPR_CXX14 explicit min_pointer(T* p) TEST_NOEXCEPT : ptr_(p) {}