net-lwip: zero terminate string with headers in wget_lwip_fill_info()

net-lwip: zero terminate string with headers in wget_lwip_fill_info()

    This patch comes as a companion to the same patch but for the legacy
    net stack. Commit 1327c2a8d6 ("net/lwip: wget: integrate struct wget_info
    into wget code") introduced function wget_fill_info() which retrieves
    the headers from the HTTP server response. As we want to parse the
    string in later patches we need to ensure that it is NUL terminated.

    We must further check that wget_info->headers in not NULL.
    Otherwise a crash occurs.

Signed-off-by: Adriano Cordova <adrianox@gmail.com>