net: http_client: Set body_start pointer unconditionally

Set `body_start` pointer regardless of the body position in the recv
buffer. In result, the pointer shall indicate correctly position of the
body for each fragment, it's also explicit now that if the pointer is
not set for a fragment, there's no body in that particular fragment.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>