fix buggy Java version check in Scaladoc tool

this is a post-2.13.1 regression, due to scala#8663

the bug was that the Java version might be e.g. "14", with
no . characters at all, causing StringIndexOutOfBoundsException

this was caught by the Scala community build

src/scaladoc/scala/tools/nsc/doc/model/MemberLookup.scala

@@ -119,24 +119,17 @@ trait MemberLookup extends base.MemberLookupBase {
   }
 
   lazy val defaultJdkUrl = {
-    if (javaVersion < 11) {
+    if (isBeforeJava11) {
       s"https://docs.oracle.com/javase/$javaVersion/docs/api"
     }
     else {
       s"https://docs.oracle.com/en/java/javase/$javaVersion/docs/api"
     }
   }
 
-  lazy val javaVersion: Int = {
-    var version = System.getProperty("java.version")
-    (if (version.startsWith("1.")) {
-      version.substring(2, 3)
-    }
-    else {
-      val dot = version.indexOf(".")
-      version.substring(0, dot)
-    }).toInt
-  }
+  lazy val isBeforeJava11: Boolean =
+    System.getProperty("java.version").split('.')
+      .headOption.exists(List("1", "9", "10").contains)
 
   override def warnNoLink = !settings.docNoLinkWarnings.value
 }