Fix issue 8431 and 8776: Size of constant string

Fixes these cases:

```cpp
void f(void) {
        const std::string msg="xyz";
        if(!msg.empty()){} // Always true
}
```

And out of bounds access:

```cpp
#include <string>
char fstr1(){const std::string s = "<a><b>"; return s[42]; }
wchar_t fwstr1(){const std::wstring s = L"<a><b>"; return s[42]; }
```

lib/valueflow.cpp

@@ -5257,7 +5257,7 @@ static void valueFlowContainerSize(TokenList *tokenlist, SymbolDatabase* symbold
     // after assignment
     for (const Scope *functionScope : symboldatabase->functionScopes) {
         for (const Token *tok = functionScope->bodyStart; tok != functionScope->bodyEnd; tok = tok->next()) {
-            if (Token::Match(tok, "[;{}] %var% = %str% ;")) {
+            if (Token::Match(tok, "%name%|;|{|} %var% = %str% ;")) {
                 const Token *containerTok = tok->next();
                 if (containerTok && containerTok->valueType() && containerTok->valueType()->container && containerTok->valueType()->container->stdStringLike) {
                     ValueFlow::Value value(Token::getStrLength(containerTok->tokAt(2)));

test/testcondition.cpp

@@ -3176,6 +3176,19 @@ private:
               "}\n");
         ASSERT_EQUALS("", errout.str());
 
+        check("void f() {\n"
+              "    const std::string x=\"xyz\";\n"
+              "    if(!x.empty()){}\n"
+              "}\n");
+        ASSERT_EQUALS("[test.cpp:3]: (style) Condition '!x.empty()' is always true\n", errout.str());
+
+        check("std::string g();\n"
+              "void f() {\n"
+              "    const std::string msg = g();\n"
+              "    if(!msg.empty()){}\n"
+              "}\n");
+        ASSERT_EQUALS("", errout.str());
+
         check("void f(int *array, int size ) {\n"
               "    for(int i = 0; i < size; ++i) {\n"
               "        if(array == 0)\n"

test/teststl.cpp

@@ -290,6 +290,15 @@ private:
                     "}\n");
         ASSERT_EQUALS("test.cpp:3:error:Out of bounds access in expression 's.begin()+x' because 's' is empty and 'x' may be non-zero.\n", errout.str());
 
+        checkNormal("char fstr1(){const std::string s = \"<a><b>\"; return s[42]; }\n"
+                    "wchar_t fwstr1(){const std::wstring s = L\"<a><b>\"; return s[42]; }\n");
+        ASSERT_EQUALS("test.cpp:1:error:Out of bounds access in 's[42]', if 's' size is 6 and '42' is 42\n"
+                      "test.cpp:2:error:Out of bounds access in 's[42]', if 's' size is 6 and '42' is 42\n", errout.str());
+
+        checkNormal("char fstr1(){const std::string s = \"<a><b>\"; return s[1]; }\n"
+                    "wchar_t fwstr1(){const std::wstring s = L\"<a><b>\"; return s[1]; }\n");
+        ASSERT_EQUALS("", errout.str());
+
         checkNormal("int f() {\n"
                     "    std::vector<int> v;\n"
                     "    std::vector<int> * pv = &v;\n"