BUG/MINOR: pattern: Do not pass len = 0 to calloc()

BUG/MINOR: pattern: Do not pass len = 0 to calloc()

The behavior of calloc() when being passed `0` as `nelem` is implementation
defined. It may return a NULL pointer.

Avoid this issue by checking before allocating. While doing so adjust the local
integer variables that are used to refer to memory offsets to `size_t`.

This issue was introced in commit f91ac19299fe216a793ba6550dca06b688b31549. This
patch should be backported together with that commit.

(cherry picked from commit b584b4475be09a20dea07f35c605487a1b73425e)
Signed-off-by: Willy Tarreau <w@1wt.eu>