Missing do_cleanups in ada_make_symbol_completion_list.

This results in an internal-warning when trying the completion
when in Ada mode:

    (gdb) b simple<TAB>
    /[...]/cleanups.c:265: internal-warning: restore_my_cleanups has found a stale cleanup
    A problem internal to GDB has been detected,
    further debugging may prove unreliable.
    Quit this debugging session? (y or n)

gdb/ChangeLog:

        * ada-lang.c (ada_make_symbol_completion_list): Make sure
        all cleanups are done before returning from this function.

gdb/testsuite/ChangeLog:

        * gdb.ada/complete.exp: Add test verifying completion using
        the "tab" key.

gdb/ada-lang.c

@@ -5841,6 +5841,7 @@ ada_make_symbol_completion_list (const char *text0, const char *word,
   struct block *b, *surrounding_static_block = 0;
   int i;
   struct block_iterator iter;
+  struct cleanup *old_chain = make_cleanup (null_cleanup, NULL);
 
   gdb_assert (code == TYPE_CODE_UNDEF);
 
@@ -5941,6 +5942,7 @@ ada_make_symbol_completion_list (const char *text0, const char *word,
     }
   }
 
+  do_cleanups (old_chain);
   return completions;
 }