The schematic doesn't even put the PMOS symbols correctly in place, it's probably rather a "design idea" than a working circuit. There are more problems like missing dead-time generation and non-optimal gate voltage range.
The stepper motor is operated with constant voltage and no option to turn off or at least reduce the motor power in idle state. If you take the circuit as is (after correcting obvious drawing faults), you only have to adjust the supply voltage for a suitable continuous motor voltage and won't necessarily need additional protection except for a fuse preventing the circuit from catching fire.