Bounds check access to Ada task state names

While looking into Ada tasking a little, I noticed that no bounds
checking is done on accesses to the Ada task state names arrays.  This
isn't a problem currently, but if the runtime ever added numbers -- or
if there was some kind of runtime corruption -- it could cause a gdb
crash.

This patch adds range checking.  It also adds a missing _() call when
printing from the 'task_states' array.