net: wget: prevent overwriting reserved memory

This introduces the valid range check to store the received
blocks using lmb. The same logic is implemented in tftp.

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>