param_cnt: reject invalid inputs instead of crashing or reporting false success

[lenzo-ka]

Fixes #65.

param_cnt turned several invalid-input conditions into either a crash or a
successful exit. This makes each one an ordinary, named initialization failure,
and defers creating the output file until a nonzero count exists.

What changes

• -segdir is required only where it is used. State and codebook counting
  need segmentations; phone counting does not. A missing -segdir is now a
  named error for state/cb and is accepted for phone, instead of passing a
  null value into corpus_set_seg_dir().
• The control file is checked. corpus_set_ctl_filename()'s result is no
  longer discarded; an unusable -ctlfn fails initialization.
• cb counting requires -ts2cbfn. Without it, mdef->cb is never
  populated and cb_cnt() would index a null map. Counting now refuses to start.
• Unknown -paramtype is rejected instead of scanning the corpus with an
  unset counting-function pointer.
• Zero counted utterances is a failure. enum_corpus() now returns the
  number of utterances that reached the counting callback (and skips an indented
  first-line comment that lineiter_start_clean() can otherwise expose as a
  control entry). A control that yields no counted utterance — empty,
  whitespace-only, or comment-only — fails instead of producing an all-zero file.
• -outputfn is opened only after enumeration produced nonzero work, so a
  failed run never leaves an empty or all-zero count file behind. A failed
  fopen() is now a nonzero exit rather than a silent write to a null stream.

Verification

param_cnt builds cleanly (no new warnings). param_cnt -paramtype state
without -segdir now prints You must specify a segmentation directory using -segdir and exits 1; -paramtype phone no longer requires -segdir. The
counting core matches the implementation carried and tested downstream, whose
boundary suite covers empty / whitespace-only / indented-comment-only controls,
a directory / unreadable file / /dev/null control, a malformed later control
line, unknown type, missing codebook mapping, output-open failure, and
successful phone counting without -segdir.

Out of scope: the cmd_ln_parse() return-contract issue (#63) and the separate
pre-existing -sentdir dereference reachable when -lsnfn is absent.