Use target.capacity() even with std::string

include/spdlog/details/os-inl.h

@@ -462,12 +462,8 @@ SPDLOG_INLINE void wstr_to_utf8buf(wstring_view_t wstr, memory_buf_t &target)
         return;
     }
 
-#ifdef SPDLOG_USE_STD_FORMAT
-    int result_size = 0;
-#else
     int result_size = static_cast<int>(target.capacity());
     if ((wstr_size + 1) * 2 > result_size)
-#endif
     {
         result_size = ::WideCharToMultiByte(CP_UTF8, 0, wstr.data(), wstr_size, NULL, 0, NULL, NULL);
     }
@@ -501,12 +497,8 @@ SPDLOG_INLINE void utf8_to_wstrbuf(string_view_t str, wmemory_buf_t &target)
         return;
     }
 
-#ifdef SPDLOG_USE_STD_FORMAT
-    int result_size = 0;
-#else
     int result_size = static_cast<int>(target.capacity());
     if (str_size + 1 > result_size)
-#endif
     {
         result_size = ::MultiByteToWideChar(CP_UTF8, MB_ERR_INVALID_CHARS, str.data(), str_size, NULL, 0);
     }