Fix Bug "double free"

[hwchen18546]

*** glibc detected *** ./ropchain: double free or corruption (!prev): 0x08a72780 ***
======= Backtrace: =========
/lib/i386-linux-gnu/libc.so.6(+0x75b12)[0xb75c7b12]
./libropchain.so(rop_chain_list_free+0x5b)[0xb770f905]
./libropchain.so(rop_chain_execve+0xd0)[0xb770e505]
./libropchain.so(rop_chain+0x130)[0xb770ddec]
./ropchain[0x80489e5]
/lib/i386-linux-gnu/libc.so.6(__libc_start_main+0xf3)[0xb756b4d3]
./ropchain[0x8048731]