mkbootimg: extract to 'second' if size not 0

Change-Id: I5f6804be6372371d51d5757f6a5700c07ed5e57f
diff --git a/mkbootimg/unpack_bootimg.py b/mkbootimg/unpack_bootimg.py
index 6b5d5d0..36a5f9f 100755
--- a/mkbootimg/unpack_bootimg.py
+++ b/mkbootimg/unpack_bootimg.py
@@ -103,10 +103,11 @@
                                  ) # header + kernel
     image_info_list.append((ramdisk_offset, ramdisk_size, 'ramdisk'))
 
-    second_offset = page_size * (
-        num_header_pages + num_kernel_pages + num_ramdisk_pages
-    )  # header + kernel + ramdisk
-    image_info_list.append((second_offset, second_size, 'second'))
+    if second_size > 0:
+        second_offset = page_size * (
+                num_header_pages + num_kernel_pages + num_ramdisk_pages
+                )  # header + kernel + ramdisk
+        image_info_list.append((second_offset, second_size, 'second'))
 
     if recovery_dtbo_size > 0:
         image_info_list.append((recovery_dtbo_offset, recovery_dtbo_size,