fix format string for size_t params

 warning: format ‘%u’ expects argument of type ‘unsigned int’,
  but argument 3 has type ‘size_t’ [-Wformat=]

when printing size_t, one has to use %zu instead of pure %u, as size_t's
size can vary depending on the architecture.

this was only partly fixed in 37be1398520db8d51d3e1b20b0cd21dc70b5cada
Merge pull request #62 from evgeni/size_t-format

fix format string for size_t params