Merge pull request #1714 from MRtrix3/dwi2mask_fslgrad_fix

dwi2mask: Fix segfault with -fslgrad

src/dwi/gradient.cpp

@@ -117,8 +117,9 @@ namespace MR
       if (bvals.cols() != bvecs.cols())
         throw Exception ("bvecs and bvals files must have same number of diffusion directions (file \"" + bvecs_path + "\" has " + str(bvecs.cols()) + ", file \"" + bvals_path + "\" has " + str(bvals.cols()) + ")");
 
-      if (bvals.cols() != header.size (3))
-        throw Exception ("bvecs and bvals files must have same number of diffusion directions as DW-image (gradients: " + str(bvecs.cols()) + ", image: " + str(header.size(3)) + ")");
+      const size_t num_volumes = header.ndim() < 4 ? 1 : header.size(3);
+      if (size_t(bvals.cols()) != num_volumes)
+        throw Exception ("bvecs and bvals files must have same number of diffusion directions as DW-image (gradients: " + str(bvecs.cols()) + ", image: " + str(num_volumes) + ")");
 
       // bvecs format actually assumes a LHS coordinate system even if image is
       // stored using RHS - x axis is flipped to make linear 3x3 part of