Added a check for max_age being greater than 0

Author: caitlin-tibbetts

src/aws_encryption_sdk/materials_managers/caching.py

@@ -108,6 +108,9 @@ def __attrs_post_init__(self):
         if self.max_bytes_encrypted > MAX_BYTES_PER_KEY:
             raise ValueError("max_bytes_encrypted cannot exceed {}".format(MAX_BYTES_PER_KEY))
 
+        if self.max_age <= 0:
+            raise ValueError("max_age cannot be less than or equal to 0")
+
         if self.backing_materials_manager is None:
             if self.master_key_provider is None:
                 raise TypeError("Either backing_materials_manager or master_key_provider must be defined")