Resolve an unsupported special case in arena_prof_tctx_set().

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
594c759f (Optimize
arena_prof_tctx_set().)