revng.api: avoid NULL dereferencing in extract

The call to `rp_container_extract_one` can return a null pointer, add a
check that allows to avoid dereferencing it if this happens.

python/revng/api/manager.py

@@ -117,7 +117,13 @@ class Manager:
             # TODO: we really should be able to provide a detailed error here
             raise RevngException("Failed to produce targets")
 
-        return {t.serialize(): t.extract() for t in targets}
+        result = {}
+        for target in targets:
+            extracted_target = target.extract()
+            if extracted_target is None:
+                raise RevngException(f"Target {target.serialize()} extraction failed")
+            result[target.serialize()] = extracted_target
+        return result
 
     def produce_target(
         self,

python/revng/api/target.py

@@ -54,8 +54,10 @@ class Target:
         _serialized = _api.rp_target_create_serialized_string(self._target)
         return make_python_string(_serialized)
 
-    def extract(self) -> str | bytes:
+    def extract(self) -> str | bytes | None:
         _buffer = _api.rp_container_extract_one(self._container._container, self._target)
+        if _buffer == ffi.NULL:
+            return None
         size = _api.rp_buffer_size(_buffer)
         data = _api.rp_buffer_data(_buffer)
         return convert_buffer(data, size, self._container.mime)