[libcxx] [test] Portability fix of std::strstreambuf constructor test.

[amakc11]

Summary

The standard does not require the constructor strstreambuf(streamsize alsize_arg = 0) leave the stream array unallocated when called with parameter alsize_arg > 0. Conformant implementations of this constructor may allocate minimal alsize_arg number of bytes forcing str() method to return non-null pointer.

[amakc11]

ping

libcxx/test/std/depr/depr.str.strstreams/depr.strstreambuf/depr.strstreambuf.cons/default.pass.cpp

 {
     {
         std::strstreambuf s;
         assert(s.str() == nullptr);
         assert(s.pcount() == 0);
     }
     {
         std::strstreambuf s(1024);
-        assert(s.str() == nullptr);
+        LIBCPP_ASSERT(s.str() == nullptr);
         assert(s.pcount() == 0);
     }
 
   return 0;
 }