Crash if no main is defined

[milabs]

trafficstars

probably should be:

if (MainFunc) createDecodeStubBlock(MainFunc, DecodeStub);

https://github.com/tsarpaul/llvm-string-obfuscator/blob/22c57d5b29063d2348da6a24a5cd507e28d9ecb3/StringObfuscator/StringObfuscator.cpp#L207

[tsarpaul]

It won't work without main because it needs to inject the decode stub somewhere. To fix this we can hook the entry point or maybe add it as an .so init function, but this is only a PoC so I won't be supporting such features. Feel free to create a pull request if you'd like.