powerpc: do not fixup NULL ptrs

The fixup routine must not fixup NULL pointers.
Problem can be seen by
 char *testfun(void) __attribute__((weak));
 char *(*myfun)(void) = testfun;

Then add
  printf("myfun:%p, &myfun:%p\n", myfun, &myfun);
before relocation and after relocation.
myfun should be NULL in both cases but it is not.

Signed-off-by: Joakim Tjernlund <Joakim.Tjernlund@transmode.se>