fix fexpr propagation error

in rare cases would show the incorrect feature expression for
an error message

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#	modified:   PartialPreprocessor/lib/xtc_2.9.1-2.3.1.jar
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#	modified:   Frontend/src/main/scala/de/fosd/typechef/Frontend.scala
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#	JavaScriptParser/
#	test.c
#	treevis/