And another warning.

(So why can't GCC or Clang be taught to warn about *all* implicit
shortenings, as MSVC does, not just 64-bit-to-32-bit shortenings?)

Change-Id: I88c0b0aa2f1b306f58952589ff8bcae17bc29768
Reviewed-on: https://code.wireshark.org/review/4676
Reviewed-by: Guy Harris <guy@alum.mit.edu>