fix #13551: Crash in checkStructMemberUsage() (danmar#7217)

Author: ludviggunne

lib/checkunusedvar.cpp

@@ -1555,7 +1555,7 @@ void CheckUnusedVar::checkStructMemberUsage()
                 tok = tok->next()->astOperand2();
                 const ValueType *valueType = tok->valueType();
 
-                if (!valueType->containerTypeToken)
+                if (!valueType || !valueType->containerTypeToken)
                     continue;
 
                 const Type *type = valueType->containerTypeToken->type();

test/testunusedvar.cpp

@@ -2047,6 +2047,15 @@ class TestUnusedVar : public TestFixture {
                                "        x = y;\n"
                                "}\n");
         ASSERT_EQUALS("", errout_str());
+
+        // #13551 - don't crash
+        checkStructMemberUsage("struct S {};\n"
+                               "void f(vector<std::pair<int, int>> p) {\n"
+                               "    for (auto [a, _] : p) {\n"
+                               "        if (a == 0) {}\n"
+                               "    }\n"
+                               "}\n");
+        ASSERT_EQUALS("", errout_str());
     }
 
     void functionVariableUsage_(const char* file, int line, const char code[], bool cpp = true) {