CodeGen: Don't dereference end() in MachineBasicBlock::CorrectExtraCFGEdges
CodeGen: Don't dereference end() in MachineBasicBlock::CorrectExtraCFGEdges

The current MachineBasicBlock might be the last block, so FallThru may
be past the end().  Use getNextNode(), which will convert to nullptr,
rather than &*++, which is invalid if we reach the end().

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@278858 91177308-0d34-0410-b5e6-96231b3b80d8

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