helpers: Add a strdup_or_die() function

Add a helper similar to string_or_die() that performs a strdup and abort
on errors. This function have the same behaviour as string_or_die with
format set to "%s".

src/helpers.c

@@ -584,6 +584,21 @@ out_fds:
 	return ret;
 }
 
+/* Return a duplicated copy of the string using strdup().
+ * Abort if there's no memory to allocate the new string.
+*/
+char *strdup_or_die(const char *const str)
+{
+	char *result;
+
+	result = strdup(str);
+	if (!result) {
+		abort();
+	}
+
+	return result;
+}
+
 void string_or_die(char **strp, const char *fmt, ...)
 {
 	va_list ap;

src/swupd.h

@@ -357,6 +357,7 @@ extern void print_manifest_files(struct manifest *m);
 extern void swupd_deinit(int lock_fd, struct list **subs);
 extern int swupd_init(int *lock_fd);
 extern void string_or_die(char **strp, const char *fmt, ...);
+char *strdup_or_die(const char *const str);
 extern void free_string(char **s);
 void update_motd(int new_release);
 void delete_motd(void);