Fix segfault in getLifetimeVariable

[serval2412]

prevent infinite recursion, eg from LibreOffice
0 0x0000555555bdc9dd in multiComparePercent (tok=0x5555570916d0, haystack=@0x7fffff7ff0a0: 0x555555d20dd9 "%name% (",
varid=<error reading variable: Cannot access memory at address 0x7fffff7feffc>) at lib/token.cpp:354
1 0x0000555555bdd113 in Token::multiCompare (tok=0x5555570916d0, haystack=0x555555d20dd9 "%name% (", varid=0) at lib/token.cpp:499
2 0x0000555555bdd6c9 in Token::Match (tok=0x5555570916d0, pattern=0x555555d20dd9 "%name% (", varid=0) at lib/token.cpp:663
3 0x0000555555c62101 in getLifetimeVariable (tok=0x5555570921a0, errorPath=empty std::__debug::list) at lib/valueflow.cpp:2651
4 0x0000555555c62194 in getLifetimeVariable (tok=0x5555570921a0, errorPath=empty std::__debug::list) at lib/valueflow.cpp:2660
5 0x0000555555c62194 in getLifetimeVariable (tok=0x5555570921a0, errorPath=empty std::__debug::list) at lib/valueflow.cpp:2660

[orbitcowboy]

Many thanks! Could you please add a regression test?

[serval2412]

@orbitcowboy: don't have time to this but you may read some details about the bug here:
http://document-foundation-mail-archive.969070.n3.nabble.com/CppCheck-Report-Failure-td4256636.html

Also, I hesitated between "return nullptr;" and "return var;"
(I must recognize I didn't test first one but perhaps I should)

[pfultz2]

I think return var is fine. A reduced test case for this:

int& f() { return f(); }

Which this PR does fix. Although, it will still crash on:

int& g(int& i) { return i; }
int& f() { return g(f()); }

I can try to fix that case.

[serval2412]

Thank you Paul for your feedback.
Don't hesitate to push a fix, I'll remove my pullrequest.

[serval2412]

Argh, I badly understood, thought nullptr was better.
Anyway, I'll close this one and let you fix this correctly with associated test.