DeclExtractor: exit early if there is nothing to do

In LLVM 9, the `CompoundStmt::replaceStmts()` call seems to write to an invalid
memory location if the body was empty. This may happen after a parse error and
might end up corrupting the program state.

This patch makes `DeclExtractor` to exit early if there is nothing to do, which
solves the aforementioned problem.

lib/Interpreter/DeclExtractor.cpp

@@ -117,6 +117,9 @@ namespace cling {
     Scope* TUScope = m_Sema->TUScope;
     llvm::SmallVector<Stmt*, 4> Stmts;
 
+    if (CS->body_empty())
+      return FD;
+
     for (CompoundStmt::body_iterator I = CS->body_begin(), EI = CS->body_end();
          I != EI; ++I) {
       DeclStmt* DS = dyn_cast<DeclStmt>(*I);