dtc: add integer overflow checks in fdt header

Protect against integer overflows caused by malformed fdt headers.

CRs-Fixed: 749977
Change-Id: I51d87038f520bc761b163d291b0138c513c69a33
Signed-off-by: Vijay Kumar Pendoti <vpendo@codeaurora.org>
Signed-off-by: Rishabh Bhatnagar <rishabhb@codeaurora.org>