Fix slicing with negative indices

Negative indices that remained negative after adding the sequence length
caused incorrect slicing.

With the default scene for example:

   bpy.context.scene.objects[-4:2]

Gave a different result to:

   tuple(bpy.context.scene.objects)[-4:2]

Clamp indices above zero so loops that step forward works as intended.

source/blender/python/intern/bpy_rna.c

@@ -2825,9 +2825,11 @@ static PyObject *pyrna_prop_collection_subscript(BPy_PropertyRNA *self, PyObject
       const Py_ssize_t len = (Py_ssize_t)RNA_property_collection_length(&self->ptr, self->prop);
       if (start < 0) {
         start += len;
+        CLAMP_MIN(start, 0);
       }
       if (stop < 0) {
         stop += len;
+        CLAMP_MIN(stop, 0);
       }
     }
 
@@ -2955,9 +2957,11 @@ static int pyrna_prop_collection_ass_subscript(BPy_PropertyRNA *self,
         Py_ssize_t len = (Py_ssize_t)RNA_property_collection_length(&self->ptr, self->prop);
         if (start < 0) {
           start += len;
+          CLAMP_MIN(start, 0);
         }
         if (stop < 0) {
           stop += len;
+          CLAMP_MIN(stop, 0);
         }
       }