a NULL first pin object is used to indicate that there are zero
of some kind of pin associated with the StateMachine. However,
mask_and_rotate wasn't checking for zero. It actually read data from
near address 0x0 and (in my case) got a nonzero mask, which then
caused a program with GPIO11 and GPIO12 as input with pull-up and no
out pins to erroneously encounter the error "pull masks conflict with
direction masks"
a3e17189a4