Clean up comments in UsefulBuf (#227)

Co-authored-by: Laurence Lundblade <lgl@securitytheory.com>
diff --git a/inc/qcbor/UsefulBuf.h b/inc/qcbor/UsefulBuf.h
index b322e39..3b7afd7 100644
--- a/inc/qcbor/UsefulBuf.h
+++ b/inc/qcbor/UsefulBuf.h
@@ -2273,9 +2273,9 @@
 
    /* The cursor is off the end of the input buffer given.
     * Presuming there are no bugs in this code, this should never happen.
-    * If it so, the struct was corrupted. The check is retained as
+    * If it is so, the struct was corrupted. The check is retained as
     * as a defense in case there is a bug in this code or the struct is
-    * corrupted.
+    * corrupted by an attacker or accidentally.
     */
    if(pMe->cursor > pMe->UB.len) {
       return 0;