Check a dict passed to ProcessorFormatter actually came from structlog.

ProcessorFormatter assumes certain attributes are attached by wrap_for_formatter
but originally only tested if it was receiving a dictionary as the log message.

This breaks if any libraries you use happen to pass dictionaries as log messages
expecting them to print using default string formatting.

Fortunately since the behavior of the LogRecord is different between the two
paths, we can resolve this by testing for the relevent attrs before choosing.

src/structlog/stdlib.py

@@ -451,7 +451,7 @@ def format(self, record):
         # Make a shallow copy of the record to let other handlers/formatters
         # process the original one
         record = logging.makeLogRecord(record.__dict__)
-        if isinstance(record.msg, dict):
+        if isinstance(record.msg, dict) and hasattr(record,"_logger") and hasattr(record,"_name"):
             # Both attached by wrap_for_formatter
             logger = record._logger
             meth_name = record._name