* Fix crashes in valueflow
http://cppcheck1.osuosl.org:8000/crash.html
For instance in http://cppcheck1.osuosl.org:8000/styx
```
==19651==ERROR: AddressSanitizer: SEGV on unknown address 0x00000000001c (pc 0x556f21abc3df bp 0x7ffc140d2720 sp 0x7ffc140d2710 T0)
==19651==The signal is caused by a READ memory access.
==19651==Hint: address points to the zero page.
#0 0x556f21abc3de in Variable::isGlobal() const ../lib/symboldatabase.h:342
#1 0x556f221f801a in valueFlowForwardVariable ../lib/valueflow.cpp:2471
#2 0x556f22208130 in valueFlowForward ../lib/valueflow.cpp:3204
#3 0x556f221e9e14 in valueFlowReverse ../lib/valueflow.cpp:1892
#4 0x556f221f1a43 in valueFlowBeforeCondition ../lib/valueflow.cpp:2200
#5 0x556f2223dbb5 in ValueFlow::setValues(TokenList*, SymbolDatabase*, ErrorLogger*, Settings const*) ../lib/valueflow.cpp:6521
#6 0x556f220e5991 in Tokenizer::simplifyTokens1(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) ../lib/tokenize.cpp:2342
#7 0x556f21d8d066 in CppCheck::checkFile(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::istream&) ../lib/cppcheck.cpp:508
#8 0x556f21d84cd3 in CppCheck::check(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) ../lib/cppcheck.cpp:192
#9 0x556f21a28796 in CppCheckExecutor::check_internal(CppCheck&, int, char const* const*) ../cli/cppcheckexecutor.cpp:884
#10 0x556f21a24be8 in CppCheckExecutor::check(int, char const* const*) ../cli/cppcheckexecutor.cpp:198
#11 0x556f22313063 in main ../cli/main.cpp:95
```
* Add test case for crash in valueflow
24211cf8b9