Add (void) casts for variables used only in assert()s.

Report from Akim Demaille.

* src/ltlvisit/snf.cc, src/ta/taexplicit.cc, src/taalgos/tgba2ta.cc,
src/tgbaalgos/dupexp.cc, src/tgbaalgos/stutter.cc: Here.

src/tgbaalgos/stutter.cc

@@ -398,6 +398,7 @@ namespace spot
 		  // No acceptance here to preserve the state-based property.
 		  i = a->new_transition(tmp, dst, one, 0U);
 		  assert(i > num_transitions);
+		  (void)i;
 		  all -= one;
 		}
 	    }