Improve output quality of real malware samples

Investigate problems in these samples. solve them, and add regression tests: [samples.zip](https://github.com/avast-tl/retdec-idaplugin/files/1674405/samples.zip)(MALWARE!).

`89442e72227b209b7fcbe467a6b202c1788b850bdfbb52fa22c7762a45b1737c`
- Strings in function calls:
```c
OutputDebugStringW(L"It was a joke about heroin, but now im going to sell heroin to your mother :P");
CreateMutexW(0, 0, L"Local\\Fabiansomware");
StrStrW(&ExistingFileName, L"winlogon.exe");
```
- Overall quality.

`20AE07CC27965D330314776C16E8FAA3FA021AB7E4C6FFC86A3593AC4010BDFE.dat`:
- Empty functions bodies: `WinMain, sub_4018E0, sub_401870`, etc.
- Overall quality.

Start with the two sample hashes and samples.zip, then reproduce the reported string-in-function-call and empty-function-body output in the plugin. Compare the generated analysis with the listed expectations and identify the relevant regression-test location; done means the samples no longer show those defects and regression tests cover them.

Written by the indexing model from the issue text.