[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