Fix a double free of the implicit variable #50
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A raw pointer to the implicit variable is returned from `interpretExprNode`. Most expressions free ValueObjects returned from `interpretExprNode` after using the value. This resulted in the implicit variable being deleted after its first use. When the scope object is cleaned up, implicit variable is deleted again, resulting in a double free. There are two simple solutions to this: 1. Set the implicit variable pointer in the scope to NULL after it's used in the expression. 2. Use a copy of the implicit variable in the expression. I chose the latter because it seemed simpler and didn't destroy any data. Hopefully it doesn't lead to memory leaks. The one interpret statement that doesn't delete expression values after use is `interpretSystemCommandExprNode`. Aside from that function, this should be a safe change.
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Attempts to fix #47
A raw pointer to the implicit variable is returned from
interpretExprNode.Most expressions free ValueObjects returned from
interpretExprNodeafterusing the value. This resulted in the implicit variable being deleted after
its first use.
When the scope object is cleaned up, implicit variable is deleted again,
resulting in a double free. There are two simple solutions to this:
expression.
I chose the latter because it seemed simpler and didn't destroy any data.
A test has also been added to catch regressions, though it only tests the
addition operator.