Fix conversion of SWATCH flags to string form

This was broken in commit [07cdd587], where 'sizeof' was used on a pointer
instead of the buffer pointed-to.

Fix this by having the caller of ircop_flags_to_str() supply a buffer and
length, since there's only two callers and both can use happily use stack
objects.

Changelog

@@ -1,5 +1,7 @@
 [Changes 1.2.2]
 
+* Fix display of the OperView SWATCH flags variable. (caf)
+
 * Update default /fset SERVER_NOTICE value to remove extra space. (caf)
 
 * Treat all NOTICEs before registration as local server notices. (caf)