relocation: Do not relocate NULL pointers.

NULL is an absolute value and should not be relocated.
After this correction code like:
 void weak_fun(void) __attribute__((weak));
 printf("weak_fun:%p\n", weak_fun);
will still print null after relocation.

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