Avoid dying if a rare bug occurs in the look ahead bucketing

StopIteration when trying to pick a bucket in a smart way.
Doing something stupid instead. Please check me.

mammoth/inputters/dataloader.py

@@ -139,19 +139,30 @@ def __iter__(self):
                         if accum:
                             yield self.collate_fn(accum)
                         break
-                    if not any(self._lens[current_bucket_idx:]):
-                        # this was the largest bucket, so we'll need to pick the next smallest instead
-                        smallest_bucket_idx = next(
-                            bucket_idx
-                            for bucket_idx in range(smallest_bucket_idx, -1, -1)
-                            if self._lens[bucket_idx] != 0
+                    try:
+                        if not any(self._lens[current_bucket_idx:]):
+                            # this was the largest bucket, so we'll need to pick the next smallest instead
+                            smallest_bucket_idx = next(
+                                bucket_idx
+                                for bucket_idx in range(smallest_bucket_idx, -1, -1)
+                                if self._lens[bucket_idx] != 0
+                            )
+                            current_bucket_idx = smallest_bucket_idx
+                        else:
+                            # there was a larger bucket, shift the index by one
+                            current_bucket_idx = next(
+                                bucket_idx
+                                for bucket_idx in range(current_bucket_idx, len(self._buckets) + 1)
+                                if self._lens[bucket_idx] != 0
+                            )
+                    except StopIteration:
+                        logger.warning(
+                            'StopIteration when trying to pick a bucket in a smart way. '
+                            'Doing something stupid instead. Please check me.'
                         )
-                        current_bucket_idx = smallest_bucket_idx
-                    else:
-                        # there was a larger bucket, shift the index by one
                         current_bucket_idx = next(
                             bucket_idx
-                            for bucket_idx in range(current_bucket_idx, len(self._buckets) + 1)
+                            for bucket_idx in range(len(self._lens))
                             if self._lens[bucket_idx] != 0
                         )
                     _ = self._choose_and_prepare_bucket(bucket_idx=current_bucket_idx)