We don't :)
It is a bit tricky. What you see is the fw_base address being dereferenced as a uint32_t pointer. This is the first uint32_t of the vector table which in turn is the starting address of the IRQ stack.
So this checks if the irq stack would be within legit RAM.
As i have moved over the IRQ stack into CCSRAM it has to check for that possibility as well (CCSRAM start at 0x10000000)