Make null pointer check in ContextualCheckBlock explicit
Make null pointer check in ContextualCheckBlock explicit

As per #549, the logic in ContextualCheckBlock(..) in main.cpp is
implictly not causing a null pointer dereference due to behavior
of VersionBitsState(..) extraction function.

This makes the check for pIndexPrev == NULL explicit and thus
also avoids tripping static analysis tools.

Closes #549.

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