BUG: Update EMSegment: Fix "comparison of pointer with integer zero" warning

This commit is similar to r24685 but applies to vtkLevelSets
instead vtkImageLevelSets

EMSegment updates:

r17129 | jcfr | 2017-05-06 00:48:04 -0400 (Sat, 06 May 2017) | 1 line

COMP: vtkLevelSets: Removed variable to fix unused-but-set-variable warning

r17128 | jcfr | 2017-05-06 00:43:27 -0400 (Sat, 06 May 2017) | 21 lines

BUG: vtkLevelSets: Fix logic error in vtkLevelSets::CheckConvergence

This commit fixes logic error reported as "-Wextra" warning. It
integrates fix similar to r17108 that was specific to vtkImageLevelSets.h

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/path/to/EMSegment/AMF/vtkLevelSets.cxx: In member function ‘unsigned char vtkLevelSets::CheckConvergence()’:
/path/to/EMSegment/AMF/vtkLevelSets.cxx:1279:24: warning: ordered comparison of pointer with integer zero [-Wextra]
if (this->u[p] <= 0) total++;
^
/path/to/EMSegment/AMF/vtkLevelSets.cxx:1284:65: warning: ordered comparison of pointer with integer zero [-Wextra]
if ((this->stored_seg[p] == ON_STORED) && (this->u[p] > 0))
^
/path/to/EMSegment/AMF/vtkLevelSets.cxx:1287:66: warning: ordered comparison of pointer with integer zero [-Wextra]
if ((this->stored_seg[p] == OFF_STORED) && (this->u[p] <= 0))
^
/path/to/EMSegment/AMF/vtkLevelSets.cxx:1291:24: warning: ordered comparison of pointer with integer zero [-Wextra]
if (this->u[p] <= 0)
^
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git-svn-id: http://svn.slicer.org/Slicer4/trunk@26006 3bd1e089-480b-0410-8dfb-8563597acbee