net: Fail instead of truncate command name in CMessageHeader

Replace the memset/strncpy dance in `CMessageHeader::CMessageHeader`
with explicit code that copies then name and asserts the length.

This removes a warning in g++ 9.1.1 and IMO makes the code more readable
by not relying on strncpy padding and silent truncation behavior.

src/protocol.h

@@ -37,6 +37,10 @@ public:
     typedef unsigned char MessageStartChars[MESSAGE_START_SIZE];
 
     explicit CMessageHeader(const MessageStartChars& pchMessageStartIn);
+
+    /** Construct a P2P message header from message-start characters, a command and the size of the message.
+     * @note Passing in a `pszCommand` longer than COMMAND_SIZE will result in a run-time assertion error.
+     */
     CMessageHeader(const MessageStartChars& pchMessageStartIn, const char* pszCommand, unsigned int nMessageSizeIn);
 
     std::string GetCommand() const;