OSV-2021-503 Use-after-poison in std::__1::unique_ptr<SkSL::Expression, std::__1::default_delete<SkSL::Expression

OSS-Fuzz report: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=31765 Crash type: Use-after-poison READ 8 Crash state: std::1::uniqueptrSkSL::Expression, std::1::defaultdeleteSkSL::Expression SkSL::DefinitionMap::getKnownDefinition SkSL::VariableReference::constantPropagate...