OPTIM: pattern: use malloc() to initialize new pat_ref struct

OPTIM: pattern: use malloc() to initialize new pat_ref struct

As mentioned in the previous commit, in _pat_ref_new(), it was not
strictly needed to explicitly assign all struct members to 0 since
the struct was allocated with calloc() which does the zeroing for us.

However, it was verified that we already initialize all fields explictly,
thus there is no reason to keep using calloc() instead of malloc(). In
fact using malloc() is less expensive, so let's use that instead now.