Fix some of the issues reported by LLVM/clang's scan-build

Corresponding to flashrom svn r722.

Signed-off-by: Stefan Reinauer <stepan@coresystems.de>

This commit fixes only some of the issues, those that were
Acked-by: Carl-Daniel Hailfinger <c-d.hailfinger.devel.2006@gmx.net>

serprog.c

@@ -95,7 +95,7 @@ static int sp_opbuf_usage = 0;
 	whether the command is supported before doing it */
 static int sp_check_avail_automatic = 0;
 
-static void sp_die(char *msg)
+static void __attribute__((noreturn)) sp_die(char *msg)
 {
 	perror(msg);
 	exit(1);