hdf5_der

[svn-r27694] HDFFV-9521: arithmetic on a void* flagged as error by AIX XL compiler
Problem: image is a *void and xlc rejected the code of image+len.
Arithmentic on a void* is not legal in C but GCC extension allows it.

Solution: define image as a *uint_8 according to how it is used in the
code.

Tested: passed in AIX ADA machine.