Check for multiplication integer overflow in CHECK_EHDR

The initial fix in r276374 is valid only for 64-bit objects. Revert it
and return an error in CHECK_EHDR if the multiplication would overflow.

The original buffer overflow issue was found with the security/afl
fuzzer and has upstream elftoolchain ticket 462. The 32-bit object issue
with r276374 found by antoine@ during an i386 exp-run.

Sponsored by: The FreeBSD Foundation