Fix handling slices with start==None

Author: msimacek

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/objects/slice/PIntSlice.java

@@ -88,7 +88,7 @@ public final int getIntStep() {
         return step;
     }
 
-    public boolean isStartIsNone() {
+    public boolean isStartNone() {
         return startIsNone;
     }
 

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/nodes/subscript/SliceLiteralNode.java

@@ -284,7 +284,11 @@ public abstract static class SliceUnpack extends PNodeWithContext {
 
         @Specialization
         SliceInfo doSliceInt(PIntSlice slice) {
-            return new SliceInfo(slice.getIntStart(), slice.getIntStop(), slice.getIntStep());
+            int start = slice.getIntStart();
+            if (slice.isStartNone()) {
+                start = slice.getIntStep() >= 0 ? 0 : Integer.MAX_VALUE;
+            }
+            return new SliceInfo(start, slice.getIntStop(), slice.getIntStep());
         }
 
         @Specialization