Fixed allocator mismatch in drain/drop.

Fixed allocator mismatch in drain/drop.

njs_chb_destroy() frees chain nodes through chain->free() and guards
against a NULL free callback.  njs_chb_drain() and the tail-freeing
path of njs_chb_drop() called njs_mp_free() directly, which is wrong
for chains initialized with NJS_CHB_CTX_INIT() where chain->free is
js_free(), and unsafe for NGX_CHB_CTX_INIT() chains where chain->free
is NULL.

Both paths now route through chain->free() with the same NULL guard
as njs_chb_destroy().