rewrite F_from_U16_BE() a little

Shouldn't be any significant change here... it's just that
aarch64-linux-gnu-g++ 7.2 segfaults when compiling the code
at head and handles this version just fine.

Change-Id: Ib291e1298aeb8a221f3be307ef13df41f43d53d2
Reviewed-on: https://skia-review.googlesource.com/c/skcms/+/225180
Auto-Submit: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
1 file changed