io: don't forget to initialise the mmap size for zero length mappings

io_posix.cpp

@@ -47,6 +47,7 @@ mapped_file::mapped_file (::cruft::posix::fd const &src, int const mflags)
     // accepted by some systems; like Linux)
     if (meta.st_size == 0) {
         m_data = nullptr;
+        m_size = 0;
         return;
     }