Fix AttributeError in LArCVReader: initialize num_entries before use

Author: francois-drielsma

src/spine/io/core/read/larcv.py

@@ -91,6 +91,7 @@ def __init__(
         # Prepare TTrees and load files
         self.trees = {}
         self.trees_ready = False
+        self.num_entries = None
         self.file_offsets = np.empty(len(self.file_paths), dtype=np.int64)
         file_counts = []
         for key in tree_keys: