UBSan: Avoid size_t variable overflow in control interface

UBSan: Avoid size_t variable overflow in control interface

The loop "if (i-- == 0) break" style construction works in practice fine
since the check against 0 is done before decrementation. However, this
hits an UBSan warning, so split that decrementation to happen as a
separate step after the check and break from the loop.

ctrl_iface.c:5086:9: runtime error: unsigned integer overflow: 0 - 1 cannot be represented in type 'size_t' (aka 'unsigned long')

Signed-off-by: Jouni Malinen <j@w1.fi>