easily result in a message body being added to a message base with no
accompanying message header: if the header data was 0-length, the hfield_dat
was not actually allocated and the applicable smbmsg_t convenience pointer was
not set accordingly. This had the side effect that if a message were to be
imported with a blank recipient ("To" field), the header was not created at
all (to support mutliple header, single body msgs, i.e. bulkmail).
The other functions that set the convenience pointers do not treat 0-length
header field data special, so this was inconsistent behavior.
Most message types (e.g. networks) don't want 0-length "to" fields, but that
shoudl be enforced somewhere else, not here.