Fix 10957: Crash on valid code (#3991)

lib/valueflow.cpp

@@ -5736,6 +5736,9 @@ struct ConditionHandler {
                     std::swap(thenValues, elseValues);
             }
 
+            if (!condTop)
+                return;
+
             if (Token::simpleMatch(condTop, "?")) {
                 Token* colon = condTop->astOperand2();
                 forward(colon->astOperand1(), cond.vartok, thenValues, tokenlist, settings);

test/testvalueflow.cpp

@@ -6442,6 +6442,9 @@ private:
                "    void f(std::array<T*, 2>& a);\n"
                "};\n";
         valueOfTok(code, "a");
+
+        code = "void f(const char * const x) { !!system(x); }\n";
+        valueOfTok(code, "x");
     }
 
     void valueFlowHang() {