Tentative bugfix for an out-of-bounds read in watershed3d.
Tentative bugfix for an out-of-bounds read in watershed3d.

dagophil/vigra,funkey/vigra,funkey/vigra,funkey/vigra,dstoe/vigra,funkey/vigra,dstoe/vigra,dstoe/vigra,dstoe/vigra,dagophil/vigra,dagophil/vigra,dstoe/vigra,dagophil/vigra,dagophil/vigra,funkey/vigra,dagophil/vigra,dstoe/vigra,funkey/vigra,dstoe/vigra,dagophil/vigra,funkey/vigra