gh-108488: Initialize JUMP_BACKWARD cache to 0, not 17

[gvanrossum]

Fixes gh-108488.

• Issue: Counter for JUMP_BACKWARD is initialized to 17 instead of 0 #108488

[gvanrossum]

(Calling out @markshannon or @brandtbucher because one of you might care about how fast the inner loop of _PyCode_Quicken() is -- this adds a branch. Though note that once we are tracking how common branches are taken, we'll get more exceptions in this loop. And hopefully the hardware CPU's branch predictor will predict this branch well.)