libfdt: fix fdt_check_full buffer overrun

fdt_check_header assumes that its argument points to a complete header
and can read data beyond the FDT_V1_SIZE bytes which fdt_check_full
can provide.

fdt_header_size can safely return a header size with FDT_V1_SIZE bytes
available and will return a usable value even for a corrupted header.

Signed-off-by: Patrick Oppenlander <patrick.oppenlander@gmail.com>
Message-Id: <20200709041451.338548-1-patrick.oppenlander@gmail.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>