Fix missing call to VbNvTeardown()

BUG=none
TEST=compiles

Change-Id: I7c917eee4a7893d535fc8f07a21410437c50b8a0
Reviewed-on: http://gerrit.chromium.org/gerrit/4956
Reviewed-by: Bill Richardson <wfrichar@chromium.org>
Tested-by: Randall Spangler <rspangler@chromium.org>

firmware/lib/vboot_api_kernel.c

@@ -703,6 +703,7 @@ VbError_t VbSelectAndLoadKernel(VbCommonParams* cparams,
 
 VbSelectAndLoadKernel_exit:
 
+  VbNvTeardown(&vnc);
   if (vnc.raw_changed)
     VbExNvStorageWrite(vnc.raw);