extract_reduce: Fix segfault on "undriven" inputs
extract_reduce: Fix segfault on "undriven" inputs

This is easily triggered when un-techmapping if the technology-specific
cell library isn't loaded. Outputs of technology-specific cells will be
seen as inputs, and nets using those outputs will be seen as undriven.
Just ignore these cells because they can't be part of a reduce chain
anyways.

SymbiFlow/yosys,YosysHQ/yosys,SymbiFlow/yosys,azonenberg/yosys,azonenberg/yosys,swallat/yosys,antmicro/yosys,antmicro/yosys,swallat/yosys,swallat/yosys,antmicro/yosys,YosysHQ/yosys,YosysHQ/yosys,swallat/yosys,YosysHQ/yosys,antmicro/yosys,swallat/yosys,SymbiFlow/yosys,SymbiFlow/yosys,azonenberg/yosys,swallat/yosys,SymbiFlow/yosys,swallat/yosys,azonenberg/yosys,cliffordwolf/yosys,SymbiFlow/yosys,cliffordwolf/yosys,SymbiFlow/yosys,swallat/yosys,azonenberg/yosys,cliffordwolf/yosys,cliffordwolf/yosys,azonenberg/yosys,azonenberg/yosys,YosysHQ/yosys,azonenberg/yosys,antmicro/yosys,YosysHQ/yosys,cliffordwolf/yosys,YosysHQ/yosys,antmicro/yosys,cliffordwolf/yosys,cliffordwolf/yosys,cliffordwolf/yosys,cliffordwolf/yosys,antmicro/yosys,swallat/yosys,YosysHQ/yosys,SymbiFlow/yosys,azonenberg/yosys,antmicro/yosys