Prevent integer-overflow in htmlSkipBlankChars() and xmlSkipBlankChars()

* HTMLparser.c:
(htmlSkipBlankChars):
* parser.c:
(xmlSkipBlankChars):
- Cap the return value at INT_MAX.
- The commit range that OSS-Fuzz listed for the fix didn't make
  any changes to xmlSkipBlankChars(), so it seems like this
  issue may still exist.

Found by OSS-Fuzz Issue 44803.

HTMLparser.c

@@ -598,7 +598,8 @@ htmlSkipBlankChars(xmlParserCtxtPtr ctxt) {
 	    if (*ctxt->input->cur == 0)
 		xmlParserInputGrow(ctxt->input, INPUT_CHUNK);
 	}
-	res++;
+	if (res < INT_MAX)
+	    res++;
     }
     return(res);
 }