[fix] check and load weight for backward w.r.t. data
[fix] check and load weight for backward w.r.t. data

fix NULL pointer when back-propagating to the data for fixed i.e. not
back-propagated weights. same issue as #1048

CZCV/s-dilation-caffe,wangg12/caffe,gnina/gnina,longjon/caffe,longjon/caffe,gnina/gnina,gnina/gnina,CZCV/s-dilation-caffe,CZCV/s-dilation-caffe,CZCV/s-dilation-caffe,gogartom/caffe-textmaps,tackgeun/caffe,tackgeun/caffe,gnina/gnina,tackgeun/caffe,gnina/gnina,wangg12/caffe,gogartom/caffe-textmaps,wangg12/caffe,gnina/gnina,tackgeun/caffe,gogartom/caffe-textmaps,gogartom/caffe-textmaps,wangg12/caffe,longjon/caffe,longjon/caffe