Check harder for URLs that are not invalid for the particular scheme.

@@ -57,6 +57,9 @@ _url_imap_init (url_t url)
   if (status)
     return status;
 
+  if(!url->host || url->query)
+    return EINVAL;
+
   /* is it pop? */
   if (strcmp ("imap", url->scheme) != 0)
     return EINVAL;

@@ -62,7 +62,7 @@ _url_pop_init (url_t url)
     return EINVAL;
 
   /* not valid in a pop url */
-  if (url->path || url->query)
+  if (url->path || url->query || !url->host)
     return EINVAL;
 
   if (url->port == 0)