valueflow.cpp: fixed -Wmaybe-uninitialized GCC warning in optimized build

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lib/valueflow.cpp: In function ‘const Token* ValueFlow::solveExprValue(const Token*, const std::function<std::vector<long long int>(const Token*)>&, Value&)’:
lib/valueflow.cpp:8405:28: warning: ‘intval’ may be used uninitialized [-Wmaybe-uninitialized]
 8405 |             value.intvalue /= intval;
      |             ~~~~~~~~~~~~~~~^~~~~~~~~
lib/valueflow.cpp:8383:21: note: ‘intval’ was declared here
 8383 |     MathLib::bigint intval;
      |                     ^~~~~~

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GCC is technically correct but the code using it will never be executed because of the logic involved.