libvncserver: avoid colour map alignment issues

[marcofortina]

Summary

Fixes the pointer aliasing/alignment issue in rfbSendSetColourMapEntries().

The previous code built a wire buffer as char[], then cast it to rfbSetColourMapEntriesMsg * and uint16_t *. That can create unaligned accesses on strict-alignment architectures and also relies on type-punning through incompatible pointer types.

Changes

• Write the SetColourMapEntries header directly into the byte buffer.
• Write 16-bit wire values through a small memcpy() helper instead of casting the byte buffer to uint16_t *.
• Keep the existing wire layout and endian conversion behavior.
• Add a NULL check for the dynamic buffer allocation path used when nColours > 256.

Validation

Tested with a minimal local CMake configuration:

cmake -S . -B build-668-patch \
  -DWITH_EXAMPLES=OFF \
  -DWITH_TESTS=ON \
  -DWITH_OPENSSL=OFF \
  -DWITH_GNUTLS=OFF \
  -DWITH_GCRYPT=OFF \
  -DWITH_SDL=OFF \
  -DWITH_GTK=OFF \
  -DWITH_QT=OFF \
  -DWITH_FFMPEG=OFF \
  -DWITH_XCB=OFF \
  -DWITH_LIBSSHTUNNEL=OFF \
  -DWITH_SYSTEMD=OFF \
  -DCMAKE_BUILD_TYPE=Debug
cmake --build build-668-patch --parallel 1
ctest --test-dir build-668-patch --output-on-failure

Result:

100% tests passed, 0 tests failed out of 5

Notes

Closes #668.