easygetopt: pass a valid enum to avoid compiler warning

"integer constant not in range of enumerated type 'CURLoption'"

Reported-by: Gisle Vanem
Bug: 6ebe63fac2 (commitcomment-42042843)

Closes #5915

lib/easygetopt.c

@@ -51,7 +51,8 @@ static struct curl_easyoption *lookup(const char *name, CURLoption id)
 
 const struct curl_easyoption *curl_easy_option_by_name(const char *name)
 {
-  return lookup(name, 0);
+  /* when name is used, the id argument is ignored */
+  return lookup(name, CURLOPT_LASTENTRY);
 }
 
 const struct curl_easyoption *curl_easy_option_by_id(CURLoption id)