[libc++][test] Portability fix of std::strstreambuf constructor test

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.

Thanks to Andrey Maksimov for the patch.

Differential Revision: https://reviews.llvm.org/D72465

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

@@ -26,7 +26,7 @@ int main(int, char**)
     }
     {
         std::strstreambuf s(1024);
-        assert(s.str() == nullptr);
+        LIBCPP_ASSERT(s.str() == nullptr);
         assert(s.pcount() == 0);
     }