Fix compile failure on win32

tests\btree_str_kv_test.cc(51) : error C2057: expected constant expression
tests\btree_str_kv_test.cc(51) : error C2466: cannot allocate an array of constant size 0
tests\btree_str_kv_test.cc(51) : error C2133: 'kv_str' : unknown size

Change-Id: I805fdca4d1c229747d9f106b93fa6b6f11e111a5