net: Sanitize message type for logging

- Use `SanitizeString` when logging message errors to make sure that the
message type is sanitized.

- For the `MESSAGESTART` error don't inspect and log header details at
all: receiving invalid start bytes makes it likely that the packet isn't
even formatted as valid P2P message. Logging the four unexpected start
bytes should be enough.

- Update `p2p_invalid_messages.py` test to check this.

Issue reported by gmaxwell.

test/functional/p2p_invalid_messages.py

@@ -37,7 +37,7 @@ VALID_DATA_LIMIT = MAX_PROTOCOL_MESSAGE_LENGTH - 5  # Account for the 5-byte len
 class msg_unrecognized:
     """Nonsensical message. Modeled after similar types in test_framework.messages."""
 
-    msgtype = b'badmsg'
+    msgtype = b'badmsg\x01'
 
     def __init__(self, *, str_data):
         self.str_data = str_data.encode() if not isinstance(str_data, bytes) else str_data
@@ -104,7 +104,7 @@ class InvalidMessagesTest(BitcoinTestFramework):
     def test_magic_bytes(self):
         self.log.info("Test message with invalid magic bytes disconnects peer")
         conn = self.nodes[0].add_p2p_connection(P2PDataStore())
-        with self.nodes[0].assert_debug_log(['HEADER ERROR - MESSAGESTART (badmsg, 2 bytes), received ffffffff']):
+        with self.nodes[0].assert_debug_log(['HEADER ERROR - MESSAGESTART received ffffffff']):
             msg = conn.build_message(msg_unrecognized(str_data="d"))
             # modify magic bytes
             msg = b'\xff' * 4 + msg[4:]