imap.c: Fixed an incorrect variable reference

Fixed an incorrect variable reference which was introduced in commit
a1701eea28 as a result of a copy and paste from SMTP/POP3.

lib/imap.c

@@ -378,7 +378,7 @@ static bool imap_endofresp(struct connectdata *conn, char *line, size_t len,
           wordlen++;
 
         /* Does the server support the STARTTLS capability? */
-        if(len >= 8 && !memcmp(line, "STARTTLS", 8))
+        if(wordlen >= 8 && !memcmp(line, "STARTTLS", 8))
           imapc->tls_supported = TRUE;
 
         /* Has the server explicitly disabled clear text authentication? */