Fix #13121 FP knownEmptyContainer for static variable (danmar#6971)

Author: chrchr-github

lib/valueflow.cpp

@@ -6599,7 +6599,7 @@ static void valueFlowContainerSize(const TokenList& tokenlist,
                 }
                 for (const ValueFlow::Value& value : values)
                     setTokenValue(tok, value, settings);
-            } else if (Token::Match(tok, "%name%|;|{|}|> %var% =") && Token::Match(tok->tokAt(2)->astOperand2(), "[({]") &&
+            } else if (Token::Match(tok, ";|{|} %var% =") && Token::Match(tok->tokAt(2)->astOperand2(), "[({]") &&
                        // init list
                        ((tok->tokAt(2) == tok->tokAt(2)->astOperand2()->astParent() && !tok->tokAt(2)->astOperand2()->astOperand2() && tok->tokAt(2)->astOperand2()->str() == "{") ||
                         // constructor

test/teststl.cpp

@@ -6792,6 +6792,14 @@ class TestStl : public TestFixture {
               "}\n",
               true);
         ASSERT_EQUALS("", errout_str());
+
+        check("void f(bool b) {\n" // #13121
+              "    static std::string s = {};\n"
+              "    for (auto c : s) {}\n"
+              "    if (b)\n"
+              "        s += \'a\';\n"
+              "}\n", true);
+        ASSERT_EQUALS("", errout_str());
     }
 
     void checkMutexes() {