Initialize a variable in all cases

Commit d167f9bc06a577d6c85b8ed6991c1efe175aae7d missed this one:
/src/qemu/ui/vnc-enc-tight.c:1483: warning: 'ret' may be used uninitialized in this function

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
