Fix: segfault when indexing into some collections with strings.

This happens when the collection's item type doesn't have a
'nameproperty' to index with.  For debug builds we error out with an
assert, since in general this shouldn't happen.  For release builds
Python will report item not found.

@@ -2374,0 +2375,4 @@
+          nameprop,
+          "Attempted to use a string to index into a collection of items with no 'nameproperty'.");
+      if (nameprop == NULL) {
+        // For non-debug builds, bail if there's no 'nameproperty' to check.