dtc: add integer overflow checks in fdt header

Protect against integer overflows caused by malformed fdt headers.

[eberman@codeaurora.org: most of changes have been fixed in 
commit f858927f ("scripts/dtc: Update to upstream version
v1.4.7-14-gc86da84d30e4"). The upstream was commit eb890c0f77dc
("libfdt: Make fdt_check_header() more thorough"))]
CRs-Fixed: 749977
Change-Id: I51d87038f520bc761b163d291b0138c513c69a33
Signed-off-by: Vijay Kumar Pendoti <vpendo@codeaurora.org>
Signed-off-by: Elliot Berman <eberman@codeaurora.org>