Fix #13224 fuzzing crash in CheckAssert::checkVariableAssignment() (danmar#7146)

Author: chrchr-github

lib/checkassert.cpp

@@ -152,6 +152,8 @@ void CheckAssert::checkVariableAssignment(const Token* assignTok, const Scope *a
     if (!assignTok->isAssignmentOp() && assignTok->tokType() != Token::eIncDecOp)
         return;
 
+    if (!assignTok->astOperand1())
+        return;
     const Variable* var = assignTok->astOperand1()->variable();
     if (!var)
         return;

test/cli/fuzz-crash_c/crash-dbd2b8fcc4fecd0b7bdde1f5938711075a35d242

@@ -0,0 +1 @@
+{assert({f=a})}