MINOR: pattern: add _pat_ref_new() helper func

MINOR: pattern: add _pat_ref_new() helper func

pat_ref_newid() and pat_ref_new() are two functions to create and
initialize a pat_ref struct based on input parameters.

Both function perform the same generic allocation and initialization
for pat_ref struct, thus there is quite a lot of code redundancy.

This is error-prone if the pat_ref init sequence has to be updated at
some point.

To reduce maintenance costs, let's add a _pat_ref_new() helper func that
takes care of the generic allocation and base initialization for pat_ref
struct.