* The caller is responsible for freeing the returned buffer. If the requested
* field is not present, or anything else goes wrong, it returns NULL.
*/
-char *rfc822_fetch_field(char *rfc822, const char *fieldname) {
+char *rfc822_fetch_field(const char *rfc822, const char *fieldname) {
char *fieldbuf = NULL;
const char *end_of_headers;
const char *field_start;