sequencer: silence -Wtautological-constant-out-of-range-compare

sequencer: silence -Wtautological-constant-out-of-range-compare

When clang compiles sequencer.c, it complains:

  sequencer.c:632:14: warning: comparison of constant 2 with
    expression of type 'const enum todo_command' is always
    true [-Wtautological-constant-out-of-range-compare]
          if (command < ARRAY_SIZE(todo_command_strings))

This is because "command" is an enum that may only have two
values (0 and 1) and the array in question has two elements.

As it turns out, clang is actually wrong here, at least
according to its own bug tracker:

  https://llvm.org/bugs/show_bug.cgi?id=16154

But it's still worth working around this, as the warning is
present with -Wall, meaning we fail compilation with "make
DEVELOPER=1".

Casting the enum to size_t sufficiently unconfuses clang. As
a bonus, it also catches any possible out-of-bounds access
if the enum takes on a negative value (which shouldn't
happen either, but again, this is a defensive check).

Signed-off-by: Jeff King <peff@peff.net>