Fix compilation by gcc-4.9

VTK fails to build during compilation by gcc-4.9 with the
following message:

CMakeFiles/vtkFiltersParallelMPI.dir/vtkDistributedDataFilter.cxx.o: In
function `memset':
/usr/include/x86_64-linux-gnu/bits/string3.h:81: warning: memset used
with constant zero length parameter; this could be due to transposed
parameters
collect2: error: ld returned 1 exit status

This patch sets if-condition before all "memsets" in
vtkDistributedDataFilter and checkes, whether the number
of bytes to be set by memset is more than 0 to escape this
error.