In "col_set_addr()", don't do anything other than clearing the column

In "col_set_addr()", don't do anything other than clearing the column
expression information if the address type is AT_NONE -
"address_to_str_buf()" panics if passed an AT_NONE address, as there's
nothing sensible one can do with them.  (A null string wouldn't be
appropriate here, as a dissector might have set the address columns to a
string.)

svn path=/trunk/; revision=8269