COMP: Fix the vtkJPEGReader setjmp clobber warning.

With -Wclobbered, gcc produced the following for vtkJPEGReader.cxx:
warning: variable ‘fp’ might be clobbered by ‘longjmp’ or ‘vfork’.
The problem was that setjmp cannot restore all variables, so 'fp'
in particular should be stored in the setjmp data struct.

Change-Id: I54eb886585f21b69a65aeaebbaa5ae22983c8ec1
Merge topic 'jpeg-setjmp-warning' into master

70c43542 COMP: Fix the vtkJPEGReader setjmp clobber warning.