network/sysctl-monitor: fix sanity check in cut_last()

This also adds basic comment about the return code.

Follow-up for 6d9ef22acd.

src/network/bpf/sysctl_monitor/sysctl-monitor.bpf.c

@@ -36,23 +36,22 @@ struct str {
 static long cut_last(u32 i, struct str *str) {
         char *s;
 
+        /* Sanity check for the preverifier */
+        if (i >= str->l)
+                return 1; /* exit from the loop */
+
         i = str->l - i - 1;
         s = str->s + i;
 
-        /* Sanity check for the preverifier */
-        if (i >= str->l)
-                return 1;
-
         if (*s == 0)
-                return 0;
+                return 0; /* continue */
 
         if (*s == '\n' || *s == '\r' || *s == ' ' || *s == '\t') {
                 *s = 0;
-
-                return 0;
+                return 0; /* continue */
         }
 
-        return 1;
+        return 1; /* exit from the loop */
 }
 
 /* Cut off trailing whitespace and newlines */