Merge #16995: Fix gcc 9 warnings

ff9c671b11d40e5d0623eff3dd12e48cbaafb34e refactor: Work around GCC 9 `-Wredundant-move` warning (Russell Yanofsky)
b837b334db5dd6232725fd2350928ff4fbd3feee net: Fail instead of truncate command name in CMessageHeader (Wladimir J. van der Laan)

Pull request description:

  Fixes all 3 from #16992 (see commits)

  - net: Fail instead of truncate command name in CMessageHeader
  - refactor: Use std::move workaround for unique_ptr upcast only when necessary

ACKs for top commit:
  practicalswift:
    ACK ff9c671b11d40e5d0623eff3dd12e48cbaafb34e -- patch looks correct
  sipa:
    utACK ff9c671b11d40e5d0623eff3dd12e48cbaafb34e
  ryanofsky:
    Code review ACK ff9c671b11d40e5d0623eff3dd12e48cbaafb34e. Looks good and seems to pass travis, modulo a timeout on one build
  hebasto:
    ACK ff9c671b11d40e5d0623eff3dd12e48cbaafb34e, tested on Fedora 31:

Tree-SHA512: 52d8c13aaf0d56f9bc546a98d7f853eae21f7e325b202fdeb2286b19a9a0ee308634c644b039f60ad8043421e382381cbf1bce58d9f807547f928621c7d245d0

1 files changed, 7 insertions, 2 deletions

diff --git a/src/protocol.cpp b/src/protocol.cpp
index e49e5523a..bd3ed25a8 100644
--- a/src/protocol.cpp
+++ b/src/protocol.cpp
@@ -85,8 +85,13 @@ 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);
 }