Fix #4019, NameError peer in pureftpd shellshock exploit

This fixes issue #4019 by changing ```#{peer}```, which is not defined to ```#{rhost}:#{rport}```.

This also explicitly disconnects in the check method to clean up.

Before:
```
msf-git (S:0 J:0) exploit(pureftpd_bash_env_exec) > rexploit
[*] Reloading module...

[*] Started reverse handler on 192.168.1.112:4444 
[-] Exploit failed: NameError undefined local variable or method `peer' for #<Msf::Modules::Mod6578706c6f69742f6d756c74692f6674702f70757265667470645f626173685f656e765f65786563::Metasploit4:0x0000000e7c4008>

[*] Exploit completed, but no session was created.
msf-git (S:0 J:0) exploit(pureftpd_bash_env_exec) 
```

After:
```
msf-git (S:0 J:0) exploit(pureftpd_bash_env_exec) > rexploit
[*] Reloading module...

[*] Started reverse handler on 192.168.1.112:4444 
[-] Exploit failed [bad-config]: 192.168.1.110:21 - Failed to store payload inside executable, please select a native payload
[*] Exploit completed, but no session was created.
msf-git (S:0 J:0) exploit(pureftpd_bash_env_exec) >
```