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>