bd_name_from_char_pointer: clean remaining memory

If name is overriden by new shorter value, we shouldn't have trashes
left. Also, get rid of overload of bd_name_copy, to make it clear when
each version is used.

Flag: exempt, added unit tests
Bug: 329458883
Test: atest net_test_types
Change-Id: I3f4d8905089b0978a98b20f73735a63d5cee7b99