cleanup: Fix warnings found by cppcheck

Fix various warnings found by cppcheck.

Signed-off-by: Lei YU <yulei.sh@bytedance.com>
Change-Id: I4db2319f5890d7aa73c8f418f610a6cba338ce24
diff --git a/image_verify.hpp b/image_verify.hpp
index 88762e1..8cda65c 100644
--- a/image_verify.hpp
+++ b/image_verify.hpp
@@ -49,7 +49,7 @@
      *
      *  @param[in] fd - File descriptor
      */
-    CustomFd(int fd) : fd(fd)
+    explicit CustomFd(int fd) : fd(fd)
     {}
 
     ~CustomFd()