Fix #12620 Crash in checkReturnByReference() (#6300)

Author: chrchr-github

lib/checkclass.cpp

@@ -3306,7 +3306,7 @@ void CheckClass::checkUselessOverride()
 }
 
 static const Variable* getSingleReturnVar(const Scope* scope) {
-    if (!scope)
+    if (!scope || !scope->bodyStart)
         return nullptr;
     const Token* const start = scope->bodyStart->next();
     const Token* const end = Token::findsimplematch(start, ";", 1, scope->bodyEnd);

test/testclass.cpp

@@ -8951,6 +8951,10 @@ class TestClass : public TestFixture {
                                "    std::string f(std::string s) { return s; }\n"
                                "};\n");
         ASSERT_EQUALS("", errout_str());
+
+        checkReturnByReference("struct S { S(); };\n" // #12620
+                               "S::S() = delete;\n");
+        ASSERT_EQUALS("", errout_str()); // don't crash
     }
 };