efi_loader: sections with zero VirtualSize

In a section header VirtualSize may be zero. This is for instance seen in
the .sbat section of shim. In this case use SizeOfRawData as section size.

Fixes: 9d30a941 ("efi_loader: don't load beyond VirtualSize")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Asherah Connor <ashe@kivikakk.ee>