Split Finish into Finish and FinishImpl to have a common place to do end of
file error checking. Use that to error on an unfinished cfi_startproc.

The error is not nice, but is already better than a segmentation fault.

llvm-svn: 147717
diff --git a/llvm/lib/MC/MCNullStreamer.cpp b/llvm/lib/MC/MCNullStreamer.cpp
index ca74663..432974f 100644
--- a/llvm/lib/MC/MCNullStreamer.cpp
+++ b/llvm/lib/MC/MCNullStreamer.cpp
@@ -94,7 +94,7 @@
                                        StringRef FileName) {}
     virtual void EmitInstruction(const MCInst &Inst) {}
 
-    virtual void Finish() {}
+    virtual void FinishImpl() {}
     
     /// @}
   };