boot: make device_path_next_node() robust against malformed zero-length nodes

boot: make device_path_next_node() robust against malformed zero-length nodes

device_path_next_node() advances by the current node's Length field,
which per the EFI device path protocol includes the 4-byte node header;
a well-formed node is therefore at least sizeof(EFI_DEVICE_PATH) bytes
long. A malformed node with Length < sizeof(EFI_DEVICE_PATH), in
particular Length == 0, makes the helper return its input pointer
unchanged.

Advance by at least sizeof(EFI_DEVICE_PATH).

Follow-up for 5080a60a719da213fa90964b76cc90bd0d1cb8de