bufref: rename *memdup() to *memdup0()

To make it clearer to readers of the code that the resulting dup also
has a null terminator. Something a "normal" memdup() does not provide.

Closes #19833

lib/bufref.c

@@ -117,7 +117,7 @@ size_t Curl_bufref_len(const struct bufref *br)
   return br->len;
 }
 
-CURLcode Curl_bufref_memdup(struct bufref *br, const void *ptr, size_t len)
+CURLcode Curl_bufref_memdup0(struct bufref *br, const void *ptr, size_t len)
 {
   unsigned char *cpy = NULL;