Check for asterisk range in CronExpression

This commit makes sure that in CronExpression, the asterisk is only used
in a range field, and is not surrounded by unexpected characters.

Closes gh-19500

spring-context/src/main/java/org/springframework/scheduling/support/BitsCronField.java

@@ -150,7 +150,7 @@ final class BitsCronField extends CronField {
 	}
 
 	private static ValueRange parseRange(String value, Type type) {
-		if (value.indexOf('*') != -1) {
+		if (value.equals("*")) {
 			return type.range();
 		}
 		else {