[efi] Treat invalid device path components as ending the path

EFI device paths generally have no externally defined length: the only
way to calculate the length is to scan the device path itself (and
therefore to implicitly assume that the path is valid).

There is no way to guard against a malformed device path (absent the
atypical existence of an external length), but we can at least prevent
infinite loops from a device path component that encodes a zero
length.

Treat any device path component with a length too short to contain the
device path header as ending the device path.  This does not prevent
invalid device paths from being accepted, but it does at least guard
against a silent system hang from an infinite loop, and ensures that
callers may safely subtract the length of the device path header from
the length of the path component without underflowing.

Signed-off-by: Michael Brown <mcb30@ipxe.org>