Remove assert from boot_status_entries

Just move error handling to caller routine.

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/boot/bootutil/src/bootutil_misc.c b/boot/bootutil/src/bootutil_misc.c
index 97066e7..35b9f15 100644
--- a/boot/bootutil/src/bootutil_misc.c
+++ b/boot/bootutil/src/bootutil_misc.c
@@ -178,11 +178,8 @@
     } else if (fap->fa_id == FLASH_AREA_IMAGE_PRIMARY(image_index) ||
                fap->fa_id == FLASH_AREA_IMAGE_SECONDARY(image_index)) {
         return BOOT_STATUS_STATE_COUNT * BOOT_STATUS_MAX_ENTRIES;
-    } else {
-        /* FIXME: make error values negative and check on caller */
-        assert(0);
-        return BOOT_EBADARGS;
     }
+    return -1;
 }
 
 uint32_t