[flang][OpenMP] Anonymous BLOCK DATA may not have Symbol at all (#165250)

This fixes https://linaro.atlassian.net/browse/LLVM-2106 and
#164815.

Author: kparzysz

flang/lib/Semantics/check-omp-structure.cpp

@@ -98,7 +98,7 @@ bool OmpStructureChecker::Enter(const parser::BlockData &x) {
   } else {
     for (const Scope &scope : context_.globalScope().children()) {
       if (scope.kind() == Scope::Kind::BlockData) {
-        if (scope.symbol()->name().empty()) {
+        if (auto *s{scope.symbol()}; !s || s->name().empty()) {
           scopeStack_.push_back(&scope);
           break;
         }

flang/test/Semantics/OpenMP/anonymous-block-data.f90

@@ -0,0 +1,11 @@
+!RUN: %flang_fc1 -fdebug-unparse -fopenmp %s | FileCheck %s
+
+! To trigger the crash, -fsyntax-only was sufficient, but when everything
+! is correct, it won't produce any output. To get something to check on
+! success, run unparse, which does run semantic checks.
+
+block data
+end
+
+!CHECK: BLOCK DATA
+!CHECK: END BLOCK DATA