Reset skinny vmexten on reload.

Make skinny reset vmexten '\0' on reload to ensure that
it is set to '\0' if the appropriate item is removed/commented in 
skinny.conf. part of ASTERISK-21037 

Reported by: snuffy
Tested by: snuffy, myself
Patches: 
    part of immed_dial_fix.diff uploaded by snuffy (license 5024)


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@380926 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/chan_skinny.c

@@ -7744,6 +7744,7 @@ static struct ast_channel *skinny_request(const char *type, struct ast_format_ca
   	}
 	memset(&bindaddr, 0, sizeof(bindaddr));
 	memset(&default_prefs, 0, sizeof(default_prefs));
+	memset(&vmexten, '\0', sizeof(vmexten));
 
 	/* Copy the default jb config over global_jbconf */
 	memcpy(&global_jbconf, &default_jbconf, sizeof(struct ast_jb_conf));