Add null pointer check to fix SEGFAULT

[knowledge4igor]

I have got the following SEGFAULT:

0   cppcheck                      	0x0000000103013ddc Token::varId() const + 12 (token.h:576)
1   cppcheck                      	0x0000000103674b58 valueFlowAfterMove(TokenList*, SymbolDatabase*, ErrorLogger*, Settings const*) + 7752 (valueflow.cpp:2963)
2   cppcheck                      	0x0000000103668856 ValueFlow::setValues(TokenList*, SymbolDatabase*, ErrorLogger*, Settings const*) + 358 (valueflow.cpp:4634)
3   cppcheck                      	0x00000001035be2ba Tokenizer::simplifyTokenList2() + 1226 (tokenize.cpp:4062)
4   cppcheck                      	0x00000001032e0bc4 CppCheck::checkFile(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, std::__1::basic_istream<char, std::__1::char_traits<char> >&) + 47380 (cppcheck.cpp:423)
5   cppcheck                      	0x00000001032d51d0 CppCheck::check(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&) + 1152 (cppcheck.cpp:83)
6   cppcheck                      	0x0000000102facc3a CppCheckExecutor::check_internal(CppCheck&, int, char const* const*) + 10458 (cppcheckexecutor.cpp:871)
7   cppcheck                      	0x0000000102fa9d86 CppCheckExecutor::check(int, char const* const*) + 502 (cppcheckexecutor.cpp:198)
8   cppcheck                      	0x0000000102f8545f main + 95 (main.cpp:95)
9   libdyld.dylib                 	0x00007fff7da1b015 start + 1

I this PR I'm proposing the fix of this problem.

[orbitcowboy]

Thanks! A test case would be nice!

[danmar]

yes the fix looks ok but a test case would be nice.

[knowledge4igor]

@orbitcowboy @danmar Ok, I will do it in the near future

[knowledge4igor]

@danmar @orbitcowboy I've added the test case (with TODO), but I'll continue research of this problem to remove TODO from the test case.

[danmar]

ok thanks! I applied these changes. Feel free to open a new PR that fix the TODO

[danmar]

For your information I plan to release Cppcheck on december 8th.. so I will not allow improvements much longer..