intel/gen_decoder: Fix parsing of small genxml file.

intel/gen_decoder: Fix parsing of small genxml file.

When using gen_spec_load_from path, only abort decoding if the read
length is 0. Previously, we were aborting if finding an EOF, even if
something was read from the file.

Also only kill the decoded file if no commands or structs were found,
and print a message in such case.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>