in rsplit with a String separator, split *after* the separator

Author: timfel

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/objects/str/StringBuiltins.java

@@ -1203,14 +1203,15 @@ public PList doSplit(String self, String sep, int maxsplit) {
             int splits = 0;
             int end = self.length();
             String remainder = self;
+            int sepLength = Math.max(1, sep.length());
             while (splits < maxsplit) {
                 int idx = remainder.lastIndexOf(sep);
 
                 if (idx < 0) {
                     break;
                 }
 
-                getAppendNode().execute(list, self.substring(idx + 1, end));
+                getAppendNode().execute(list, self.substring(idx + sepLength, end));
                 end = idx;
                 splits++;
                 remainder = remainder.substring(0, end);