tcp: detect invalid window scale shift value

According to RFC 1323, the window scale shift value must not exceed 14.
Detect this and cap at 14 to prevent undefined behavior (shifting by a
too large value).

Caught by `clang -fsanitize=undefined`.

Change-Id: I1acad252b86c7f23e497575b48d9496346327e00
Reviewed-on: https://code.wireshark.org/review/3312
Reviewed-by: Michael Mann <mmann78@netscape.net>