scripts: get_abi.pl: produce an error if the ref tree is broken

scripts: get_abi.pl: produce an error if the ref tree is broken

The logic under graph_add_file should create, for every entry, a
__name name array for all entries of the tree. If this fails, the
symlink parsing will break.

Add an error if this ever happens.

While here, improve the output of data dumper to be more
compact and to avoid displaying things like $VAR1=.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/e7dd4d70e206723455d50c851802c8bb6c34941d.1632750315.git.mchehab+huawei@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>