A macro without any assignments is considered an invalid macro by the
attributes cache and is thus not getting added to the macro map at all.
But as `git_attr_cache__insert_macro` returns success with neither
free'ing nor adopting the macro into its map, this will cause a memory
leak.

Fix this by freeing the macro in the function if it's not going to be
added. This is perfectly fine to do, as callers assume that the
attrcache will have the macro adopted on success anyway.