builtin/apply: move 'state' check into check_apply_state()

builtin/apply: move 'state' check into check_apply_state()

To libify the apply functionality we should provide a function
to check that the values in a 'struct apply_state' instance are
coherent. Let's move the code to do that into a new
check_apply_state() function.

Reviewed-by: Stefan Beller <redacted>
Signed-off-by: Christian Couder <redacted>
Signed-off-by: Junio C Hamano <redacted>