Skip the fallback lookup by parameter index without record offsets.

Fixes the test failure introduced with previous commit.

Author: acolomb

canopen/pdo/base.py

@@ -175,10 +175,12 @@ def __getitem__(self, key: int) -> PdoMap:
         try:
             return self.maps[key]
         except KeyError:
-            with contextlib.suppress(KeyError):
-                return self.maps[key + 1 - self.map_offset]
-            with contextlib.suppress(KeyError):
-                return self.maps[key + 1 - self.com_offset]
+            if self.map_offset:
+                with contextlib.suppress(KeyError):
+                    return self.maps[key + 1 - self.map_offset]
+            if self.com_offset:
+                with contextlib.suppress(KeyError):
+                    return self.maps[key + 1 - self.com_offset]
             raise
 
     def __iter__(self) -> Iterator[int]: