fix fsl_anat nonlinreg failure

This bug was originally reported by Chris Rorden.

The current fsl_anat will always fail if run with --nonlinreg - this is unfortunate as only T1 scans can be used with fnirt, and these are often not available for clinical datasets.

To replicate:

fsl_anat -i T1.nii.gz --nononlinreg

produces:

...
Image Exception : #63 :: No image files match: T1_biascorr_brain
terminate called after throwing an instance of 'std::runtime_error'
  what():  No image files match: T1_biascorr_brain

the fix is to move the else statement for brain extraction to match the if [ $do_nonlinreg = yes ]; then block.