[clang][dataflow] HTMLLogger: Don't crash if CFG contains unreachable blocks.

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D156411

Author: martinboehme

clang/lib/Analysis/FlowSensitive/HTMLLogger.cpp

@@ -462,8 +462,9 @@ class HTMLLogger : public Logger {
       GraphS << "  " << blockID(I) << " [id=" << blockID(I) << "]\n";
     for (const auto *Block : CFG) {
       for (const auto &Succ : Block->succs()) {
-        GraphS << "  " << blockID(Block->getBlockID()) << " -> "
-               << blockID(Succ.getReachableBlock()->getBlockID()) << "\n";
+        if (Succ.getReachableBlock())
+          GraphS << "  " << blockID(Block->getBlockID()) << " -> "
+                 << blockID(Succ.getReachableBlock()->getBlockID()) << "\n";
       }
     }
     GraphS << "}\n";