tree-wide: use memmem_safe()

Let's be paranoid and do something useful if we operate with empty
haystack/needle. This doesn't actually fix anything, as the places as
far as I can see check for non-emptyness already beforehand, but I will
sleep safer at night, if we don't even allow the trap to be fallen in,
ever, even if the code is changed sooner or later.

src/boot/bootctl.c

@@ -163,7 +163,7 @@ static int get_file_version(int fd, char **v) {
         if (!s)
                 goto finish;
 
-        e = memmem(s, st.st_size - (s - buf), " ####", 5);
+        e = memmem_safe(s, st.st_size - (s - buf), " ####", 5);
         if (!e || e - s < 3) {
                 r = log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Malformed version string.");
                 goto finish;