manifest: validate() fixes

We need to be a bit more careful with our types.

For example, the string "not-a-manifest" can be loaded as YAML, and
evaluates to... itself. And the string 'manifest' sure is in the
string 'not-a-manifest', so the "'manifest' not in data" check in
validate() doesn't catch the invalidity.

Then we try to index ['manifest'] in a string, which doesn't work,
because string indexes must be integers. Boom.

Fix it with extra type checking.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>

src/west/manifest.py

@@ -79,7 +79,12 @@ def validate(data):
     :param data: YAML manifest data as a string or object
     '''
     if isinstance(data, str):
+        as_str = data
         data = yaml.safe_load(data)
+        if not isinstance(data, dict):
+            raise MalformedManifest(f'{as_str} is not a YAML dictionary')
+    elif not isinstance(data, dict):
+        raise TypeError(f'data type {type(data)} must be str or dict')
 
     if 'manifest' not in data:
         raise MalformedManifest('manifest data contains no "manifest" key')