bufr_descriptor_set_svalue() doesn't work on "fresh" descriptors

[cpb]

Sample code:

   BufrDescriptor* d = bufr_create_descriptor( tables, desc );
   bufr_descriptor_set_svalue(d, value);
   fprintf(stderr,"set string to '%s' ('%s')\n", bufr_descriptor_get_svalue(d, &l),value);

The output ends up being an empty string rather than what we're assigning.

the root of the problem is that bufr_create_descriptor() does a lookup in the table and sets d->encoding.type, but that's all it sets in the encoding section. Well, bufr_descriptor_set_svalue() comes along and falls down to:

  bufr_value_set_string( cb->value, sval, cb->encoding.nbits/8 );

which, obviously, is going to fail miserably since cb->encoding.nbits==0.

Proper fix is to ensure that bufr_create_descriptor() fully defines the encoding based on the table entry.