handle off the end start in jvm's index/rindex

Author: TimToady

src/vm/jvm/runtime/org/perl6/nqp/runtime/Ops.java

@@ -3548,6 +3548,7 @@ public static SixModelObject split(String delimiter, String string, ThreadContex
     }
 
     public static long indexfrom(String string, String pattern, long fromIndex) {
+	if (fromIndex > string.length()) { return -1; }
         return string.indexOf(pattern, (int)fromIndex);
     }
 
@@ -3556,6 +3557,7 @@ public static long rindexfromend(String string, String pattern) {
     }
 
     public static long rindexfrom(String string, String pattern, long fromIndex) {
+	if (fromIndex > string.length()) { return -1; }
         return string.lastIndexOf(pattern, (int)fromIndex);
     }