Fix #14437 fuzzing crash (null-pointer-use) in ValueFlowAnalyzer::assume() (cppcheck-opensource#8730)

Author: chrchr-github

lib/tokenize.cpp

@@ -6920,7 +6920,7 @@ Token *Tokenizer::simplifyAddBracesToCommand(Token *tok)
             // before the "while"
             if (tokEnd) {
                 tokEnd=tokEnd->next();
-                if (!tokEnd || tokEnd->str()!="while") // no while
+                if (!Token::simpleMatch(tokEnd, "while (") || !Token::simpleMatch(tokEnd->linkAt(1), ") ;")) // no while
                     syntaxError(tok);
             }
         }

test/cli/fuzz-crash/crash-b4bd9ce1af8a423d15d034f42c7b99dc0f8a0a7d

@@ -0,0 +1 @@
+d o(n c){do if(c){}while(0)n}