Cleanup: eliminate implicit sign-extension

Coverity reported:

CID 1321723 (#1 of 1): Unintended sign extension
(SIGN_EXTENSION)sign_extension: Suspicious implicit sign extension:
elf->ehdr->e_shentsize with type unsigned short (16 bits, unsigned) is
promoted in index * elf->ehdr->e_shentsize to type int (32 bits,
signed), then sign-extended to type unsigned long (64 bits, unsigned).
If index * elf->ehdr->e_shentsize is greater than 0x7FFFFFFF, the upper
bits of the result will all be 1.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>