check before accessing possibly null node

[danix800]

Summary

Testcase: (main.cpp)

int foo(int i);

int main() {
  int i = 1;
  while (i-- >= 0) {
    return 3 / foo(i);
  }
}

run with

clang -cc1 -analyze -analyzer-checker=debug.DumpDominators main.cpp

triggers nullptr dereference.

clang/include/clang/Analysis/Analyses/Dominators.h

llvm::errs() << "Immediate " << (IsPostDom ? "post " : "")
                  << "dominance tree (Node#,IDom#):\n";
     for (CFG::const_iterator I = cfg->begin(),
         E = cfg->end(); I != E; ++I) {
 
       assert(*I &&
              "LLVM's Dominator tree builder uses nullpointers to signify the "
              "virtual root!");
 
-      DomTreeNode *IDom = DT.getNode(*I)->getIDom();
+      auto *Node = DT.getNode(*I);
+      if (Node == nullptr) {
+        continue;
+      }
+
+      DomTreeNode *IDom = Node->getIDom();
       if (IDom && IDom->getBlock())
         llvm::errs() << "(" << (*I)->getBlockID()
                      << ","
                      << IDom->getBlock()->getBlockID()
                      << ")\n";
       else {
         bool IsEntryBlock = *I == &(*I)->getParent()->getEntry();
         bool IsExitBlock = *I == &(*I)->getParent()->getExit();