Update indirect.c

Implement @whot suggestion for coding style & shorter test.
GetShift(mask) will return 0 for a mask of both 0 and -1.
The assumption is that if mask == 0, then the corresponding bits for which we're 
calculating the shift, are also 0. 
3 jobs for dan/GetShiftFix in 6 minutes and 39 seconds (queued for 1 second)
Status Name Job ID Coverage
  Docker Image
passed debian-testing #1127552

00:01:15

 
  Build And Test
passed autotools-build-and-test #1127553

00:05:23

passed meson-build-and-test #1127554

00:03:07