[BOLT][PR] In some cases DB could be nullptr

Summary:
When processing binary with -debug mode in some cases, BD could be nullptr. It will be better to fail later on assert than here with segfault.
Closes https://github.com/facebookincubator/BOLT/pull/18
GitHub Author: Alexander Gryanko <xpahos@gmail.com>

(cherry picked from FBD8650719)

bolt/src/BinaryFunction.cpp

@@ -1017,8 +1017,9 @@ void BinaryFunction::disassemble(ArrayRef<uint8_t> FunctionData) {
     auto *TargetSymbol =
         BC.getOrCreateGlobalSymbol(TargetAddress, 0, 0, "DATAat");
     DEBUG(if (opts::Verbosity >= 2) {
+      auto SectionName = BD ? BD->getSectionName() : "<unknown>";
       dbgs() << "Created DATAat sym: " << TargetSymbol->getName()
-             << " in section " << BD->getSectionName() << "\n";
+             << " in section " << SectionName  << "\n";
     });
     return TargetSymbol;
   };