Fix some cases where we're shifting a signed 1 left.

Fix some cases where we're shifting a signed 1 left.

Shift 1U instead, to make sure it's unsigned; the result of, for
example, the result of shifting a signed value left is undefined if the
value times 2^{shift count} doesn't fit in the *signed* type of the
shifted value.  That means, in particular, that the result of shifting 1
left by {number of bits in an int - 1} is undefined.  (In *practice*,
it'll probably be -2^32, with the bit you want set, but that's not
guaranteed, and GCC 5.1 seems not to like it.)

Change-Id: I0d27565c382a04ceda9eec65f45a430ceb74cf53
Reviewed-on: https://code.wireshark.org/review/8255
Reviewed-by: Guy Harris <guy@alum.mit.edu>