Zero initialize uses of instr_time about to trigger compiler warnings
These are all not necessary from a correctness POV. However, in the near
future instr_time will be simplified to an int64, at which point gcc would
otherwise start to warn about the changed places.
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/
20230116023639.rn36vf6ajqmfciua@awork3.anarazel.de