libvncserver: avoid deferUpdateTime overflow

[marcofortina]

Summary

Fixes a potential integer overflow in the deferred update timeout calculation.

The server stores deferUpdateTime as milliseconds, but the event loop converts it to microseconds with an int * int expression before passing it to select(). Large values, for example from -deferupdate, can overflow before the value is widened.

Changes

• Add a small helper to convert deferUpdateTime from milliseconds to microseconds safely.
• Treat non-positive values as a zero timeout, preserving the existing immediate-processing behavior.
• Clamp excessively large values to avoid overflow before passing the timeout to the socket wait helper.
• Reuse the helper in both rfbProcessEvents() and rfbRunEventLoop().

Validation

Tested with a minimal local CMake configuration:

cmake -S . -B build-701-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-701-patch --parallel 1
ctest --test-dir build-701-patch --output-on-failure

Result:

100% tests passed, 0 tests failed out of 5

Notes

Closes #701.