net: lib: http: Fix invalid pointer body_start

The body_start field at http_client_ctx.rsp is used to check if this
fragment contains (a part of) headers or not.

If the device recived more than one fragment in one http response,
may cause re-use of the result buffer in function on_body().

Once the device re-use the result buffer, the body_start that point
to this buffer address will no longer be valid.

Signed-off-by: Ding Tao <miyatsu@qq.com>