tools:iio:iio_utils: fix allocation handling

In build_channel_array(), count needs to be decreased in more places since
current->name and current->generic_name would be freed on the error path,
although they have not been allocated, yet.
This also requires to free current->name, when it is allocated, but
current->generic_name is not yet allocated.

Signed-off-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>