Fixes disconnected ahelpers runtime (#4668)

# About the pull request

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If client disconnected, due to hard del, instances get replaced by null.
This still goes through the timer CALLBACK somehow (?), causing
invocation of the proc with src nulled out.

# Explain why it's good for the game
Every time this happens are two different runtimes, one for add_verb and
one for deltimer.

# Testing Photographs and Procedure
Untested


# Changelog

Internal change

code/modules/admin/verbs/adminhelp.dm

@@ -753,6 +753,8 @@ GLOBAL_DATUM_INIT(ahelp_tickets, /datum/admin_help_tickets, new)
 //
 
 /client/proc/giveadminhelpverb()
+	if(!src)
+		return
 	add_verb(src, /client/verb/adminhelp)
 	deltimer(adminhelptimerid)
 	adminhelptimerid = 0