Fix consistency check for case of pointer comparison of external allocated and NULL pointer

SMG consistency check fails on test/programs/smg_External_Allocation_Test/undefinedPointerFromLongValue-5.c because of replacing edge to external allocated memory without edges from it with NULL.

SMGState.identifyEqualValues() -> heap.replaceValue(pKnownVal1, pKnownVal2);

Problem with git commit aefb5e3c