utils.crypto: Fix the broken algorithm check in hash_file

Fix the broken algorithm check in the hash_file function, it
should return `None` if the given hash algorithm is unknown.
Signed-off-by: NXu Han <xuhan@redhat.com>

avocado/utils/crypto.py

@@ -61,6 +61,7 @@ def hash_file(filename, size=None, algorithm="md5"):
         hash_obj = hash_wrapper(algorithm=algorithm)
     except ValueError:
         logging.error("Unknown hash algorithm %s, returning None", algorithm)
+        return None
 
     with open(filename, 'rb') as file_to_hash:
         while size > 0: