Add arena_prof_tctx_reset() and use it instead of arena_prof_tctx_set() when resetting the tctx pointer during reallocation, which happens whenever an originally sampled reallocated object is not sampled during reallocation. This regression was introduced by 594c759f37c301d0245dc2accf4d4aaf9d202819 (Optimize arena_prof_tctx_set().)