diffcore-break: guard against NULLed queue entries in merge loop

The outer loop in `diffcore_merge_broken()` sets `q->queue[j]` to NULL when it merges a broken pair back together, and has a NULL check to skip such entries on subsequent iterations. The inner loop, however, lacks this guard: when it scans forward looking for a matching peer, it can encounter a slot that was NULLed by a previous outer-loop iteration and dereference it unconditionally. In practice this requires at least two broken pairs whose peers both survive rename/copy detection and appear later in the queue, which is rare but not impossible. Add the same `if (!pp) continue` guard to the inner loop. Pointed out by Coverity. Assisted-by: Claude Opus 4.6 Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>