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/loader.c b/boot/bootutil/src/loader.c
index fc5d09c..e01d00e 100644
--- a/boot/bootutil/src/loader.c
+++ b/boot/bootutil/src/loader.c
@@ -577,6 +577,9 @@
 
     off = boot_status_off(fap);
     max_entries = boot_status_entries(BOOT_CURR_IMG(state), fap);
+    if (max_entries < 0) {
+        return BOOT_EBADARGS;
+    }
 
     found = 0;
     found_idx = 0;