boot: Fix log reporting magic from incorrect header

Magic for different header was reported than the one that had failed
the check.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>
diff --git a/boot/bootutil/src/loader.c b/boot/bootutil/src/loader.c
index f216dca..e977d65 100644
--- a/boot/bootutil/src/loader.c
+++ b/boot/bootutil/src/loader.c
@@ -2027,7 +2027,7 @@
          */
         if (BOOT_IMG(state, BOOT_PRIMARY_SLOT).hdr.ih_magic != IMAGE_MAGIC) {
             BOOT_LOG_ERR("bad image magic 0x%lx; Image=%u", (unsigned long)
-                         &boot_img_hdr(state,BOOT_PRIMARY_SLOT)->ih_magic,
+                         BOOT_IMG(state, BOOT_PRIMARY_SLOT).hdr.ih_magic,
                          BOOT_CURR_IMG(state));
             rc = BOOT_EBADIMAGE;
             goto out;