minimal_xname() fix (trunk only)

minimal_xname() fix (trunk only)

     ansimpleoname() and siblings always reported statue and figurine
type to be "of a giant ant" because the corpsenm field was left as 0.
Explicitly set it to -1 in the minimal object and teach xname() to leave
off the monster type in that situation, yielding just "a statue" or "a
figurine".  [It's tempting to classify this as an xname() bug since other
object types which use corpsenm do so in doname().]  No fixes entry; this
is post-3.4.3 code.