metadata/maven: Ignore ill-formed xml instead of failing

This fixes error instances [1]

[1] https://sentry.softwareheritage.org/share/issue/a5869f724a244fdab2e1cb0303a12fc1/

swh/indexer/metadata_dictionary/maven.py

-# Copyright (C) 2018-2019  The Software Heritage developers
+# Copyright (C) 2018-2021  The Software Heritage developers
 # See the AUTHORS file at the top-level directory of this distribution
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
@@ -36,6 +36,9 @@ class MavenMapping(DictMapping, SingleFileMapping):
             # unknown encoding or multi-byte encoding
             self.log.warning("Error detecting XML encoding from %s", self.log_suffix)
             return None
+        if not isinstance(d, dict):
+            self.log.warning("Skipping ill-formed XML content: %s", content)
+            return None
         metadata = self._translate_dict(d, normalize=False)
         metadata[SCHEMA_URI + "codeRepository"] = self.parse_repositories(d)
         metadata[SCHEMA_URI + "license"] = self.parse_licenses(d)