Distribution of uminus node

[vmiheer]

If the expression is of the form:

a = -b * _tmp_xx;

Stringify is not defined for UMINUS node. Consequently, lars fails to detect -b as coef_node and end up trying to create a binary node with one of the operands as nullptr.
Two ways this could be avoided:

1. Disable distribution of coef_node is not found (is nullptr).
   1840eba does this.
2. Extend lars to include this case.