net: initialize nMessageSize to max uint32_t instead of -1

nMessageSize is uint32_t and is set to -1. This will warn with
-fsanitize=implicit-integer-sign-change.

src/protocol.cpp

@@ -91,7 +91,6 @@ CMessageHeader::CMessageHeader()
 {
     memset(pchMessageStart, 0, MESSAGE_START_SIZE);
     memset(pchCommand, 0, sizeof(pchCommand));
-    nMessageSize = -1;
     memset(pchChecksum, 0, CHECKSUM_SIZE);
 }