Update rtkHisImageIO.cxx
Update rtkHisImageIO.cxx

Change header to unsigned char array.

When header bytes are read as signed char, a bounding rectangle size of 128 is cast to -128, and leads to errors when subsequently attempting to define an image with negative dimensions.
fabienmomey/RTK,fabienmomey/RTK,fabienmomey/RTK,fabienmomey/RTK,SimonRit/RTK,fabienmomey/RTK,SimonRit/RTK,SimonRit/RTK,SimonRit/RTK,fabienmomey/RTK,fabienmomey/RTK