Replace malloc+memset by calloc in render_expired.c

This improves startup time of render_expired, when using --max-zoom=20, as a
2GB array is initialized to 0.

On some trial, the following command switches from 1,5 seconds to ~0 seconds, which is quite noticeable when trying to expire 20 styles every minutes. The same gain is visible on an expires.list generated by a 5 minutes diff.
```
  echo "" | time /usr/bin/render_expired --map admin2 --min-zoom=12 --touch-from=12 --max-zoom=20
```