Don't shift signed values left.

Don't shift signed values left.

That's not valid in C99, at least, if the value is negative or if the
shift count is the number of bits in the value - 1, and we might get
compile-time or run-time complaints about that.

Also, make bit masks unsigned; to quote a run-time error reported in

	https://www.wireshark.org/lists/wireshark-dev/201504/msg00084.html

"left shift of 1 by 31 places cannot be represented in type 'int'", so
use type "unsigned int" instead, by shifting 1U rather than 1 left.

Change-Id: I62220808058cb93f83329c1916b888a2067d524c
Reviewed-on: https://code.wireshark.org/review/8254
Reviewed-by: Guy Harris <guy@alum.mit.edu>