AssertionError at DPLLEngine.java:1514 (incomplete fix)

[rainoftime]

Related and fixed #76

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Hi, for the following formulas,
xx.txt
1514.txt

SMTInterpol (commit 7625fe2) throws an AssertionError

Exception in thread "main" java.lang.AssertionError
        at de.uni_freiburg.informatik.ultimate.smtinterpol.dpll.DPLLEngine.generateLevel0Proof(DPLLEngine.java:1514)
        at de.uni_freiburg.informatik.ultimate.smtinterpol.dpll.DPLLEngine.setLiteral(DPLLEngine.java:457)
        at de.uni_freiburg.informatik.ultimate.smtinterpol.dpll.DPLLEngine.propagateClauses(DPLLEngine.java:402)
        at de.uni_freiburg.informatik.ultimate.smtinterpol.dpll.DPLLEngine.propagateInternal(DPLLEngine.java:243)
        at de.uni_freiburg.informatik.ultimate.smtinterpol.dpll.DPLLEngine.solve(DPLLEngine.java:1148)
        at de.uni_freiburg.informatik.ultimate.smtinterpol.smtlib2.SMTInterpol$CheckType$1.check(SMTInterpol.java:113)
        at de.uni_freiburg.informatik.ultimate.smtinterpol.smtlib2.SMTInterpol.checkSatAssuming(SMTInterpol.java:476)
        at de.uni_freiburg.informatik.ultimate.smtinterpol.smtlib2.Parser$Action$.CUP$do_action(Parser.java:2916)
        at de.uni_freiburg.informatik.ultimate.smtinterpol.smtlib2.Parser.do_action(Parser.java:1311)
        at com.github.jhoenicke.javacup.runtime.LRParser.parse(Unknown Source)
        at de.uni_freiburg.informatik.ultimate.smtinterpol.smtlib2.ParseEnvironment.parseStream(ParseEnvironment.java:154)
        at de.uni_freiburg.informatik.ultimate.smtinterpol.smtlib2.ParseEnvironment.parseScript(ParseEnvironment.java:133)
        at de.uni_freiburg.informatik.ultimate.smtinterpol.smtlib2.SMTLIB2Parser.run(SMTLIB2Parser.java:37)
        at de.uni_freiburg.informatik.ultimate.smtinterpol.Main.main(Main.java:180)