Fix nil RangeWalker length for invalid range in exploit RHOSTS

[wvu]

reset returns false and doesn't initialize length. This just restores the to_i that was present before.

Before

msf5 exploit(unix/webapp/drupal_restws_unserialize) > set rhosts .
rhosts => .
msf5 exploit(unix/webapp/drupal_restws_unserialize) > run
[-] Error while running command run: undefined method `>' for nil:NilClass

Call stack:
/rapid7/metasploit-framework/lib/msf/ui/console/command_dispatcher/exploit.rb:173:in `cmd_exploit'
/rapid7/metasploit-framework/lib/rex/ui/text/dispatcher_shell.rb:523:in `run_command'
/rapid7/metasploit-framework/lib/rex/ui/text/dispatcher_shell.rb:474:in `block in run_single'
/rapid7/metasploit-framework/lib/rex/ui/text/dispatcher_shell.rb:468:in `each'
/rapid7/metasploit-framework/lib/rex/ui/text/dispatcher_shell.rb:468:in `run_single'
/rapid7/metasploit-framework/lib/rex/ui/text/shell.rb:151:in `run'
/rapid7/metasploit-framework/lib/metasploit/framework/command/console.rb:48:in `start'
/rapid7/metasploit-framework/lib/metasploit/framework/command/base.rb:82:in `start'
./msfconsole:49:in `<main>'
msf5 exploit(unix/webapp/drupal_restws_unserialize) >

After

msf5 exploit(unix/webapp/drupal_restws_unserialize) > set rhosts .
rhosts => .
msf5 exploit(unix/webapp/drupal_restws_unserialize) > run

[-] Exploit failed: The following options failed to validate: RHOSTS.
[*] Exploit completed, but no session was created.
msf5 exploit(unix/webapp/drupal_restws_unserialize) >

Updates #11497. Fixes #12193.

[wvu]

jEnKiNs tEsT tHiS pLeAsE

[busterb]

Good change

[busterb]

Release Notes

This replaces a backtrace with a friendly error message when the user specifies an invalid value for RHOSTS in an exploit module.

[wvu]

Thanks!