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>
diff --git a/arch/powerpc/cpu/ppc4xx/start.S b/arch/powerpc/cpu/ppc4xx/start.S
index 03bde4d..87caea1 100644
--- a/arch/powerpc/cpu/ppc4xx/start.S
+++ b/arch/powerpc/cpu/ppc4xx/start.S
@@ -1608,10 +1608,12 @@
 	beq	4f
 3:	lwzu	r4,4(r3)
 	lwzux	r0,r4,r11
+	cmpwi	r0,0
 	add	r0,r0,r11
 	stw	r10,0(r3)
+	beq-	5f
 	stw	r0,0(r4)
-	bdnz	3b
+5:	bdnz	3b
 4:
 clear_bss:
 	/*