Fix deadloop if header not matched

If end-delimiter of header if not found within the page the loop will never
stop. Only to be terminated by the timeout alarm trigger.

Technically the response is received, yet it might not be valid.

plugins/check_http.c

@@ -1205,7 +1205,7 @@ check_http (void)
 
   /* find header info and null-terminate it */
   header = page;
-	for (;;) {
+	while (strlen(page)) {
 		if (!strncmp(page, "\r\n\r\n", 4) || !strncmp(page, "\n\n", 2))
 		 break;
 		while (*page == '\r' || *page == '\n') { ++page; }