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.cpp

@@ -126,8 +126,13 @@ CMessageHeader::CMessageHeader(const MessageStartChars& pchMessageStartIn)
 CMessageHeader::CMessageHeader(const MessageStartChars& pchMessageStartIn, const char* pszCommand, unsigned int nMessageSizeIn)
 {
     memcpy(pchMessageStart, pchMessageStartIn, MESSAGE_START_SIZE);
-    memset(pchCommand, 0, sizeof(pchCommand));
-    strncpy(pchCommand, pszCommand, COMMAND_SIZE);
+
+    // Copy the command name, zero-padding to COMMAND_SIZE bytes
+    size_t i = 0;
+    for (; i < COMMAND_SIZE && pszCommand[i] != 0; ++i) pchCommand[i] = pszCommand[i];
+    assert(pszCommand[i] == 0); // Assert that the command name passed in is not longer than COMMAND_SIZE
+    for (; i < COMMAND_SIZE; ++i) pchCommand[i] = 0;
+
     nMessageSize = nMessageSizeIn;
     memset(pchChecksum, 0, CHECKSUM_SIZE);
 }

src/protocol.h

@@ -33,7 +33,11 @@ class CMessageHeader
 public:
     typedef unsigned char MessageStartChars[MESSAGE_START_SIZE];
 
-    CMessageHeader(const MessageStartChars& pchMessageStartIn);
+    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;