Fixes Disconnected Observers runtiming HUD (#4667)

# About the pull request

<!-- Remove this text and explain what the purpose of your PR is.

Mention if you have tested your changes. If you changed a map, make sure
you used the mapmerge tool.
If this is an Issue Correction, you can type "Fixes Issue #169420" to
link the PR to the corresponding Issue number #169420.

Remember: something that is self-evident to you might not be to others.
Explain your rationale fully, even if you feel it goes without saying.
-->

A simple runtime job... Login() to an observed mob with a disconnected
observer will crash the inventory HUD proc because it has no client.

# Explain why it's good for the game
Less runtimes

# Testing Photographs and Procedure
Tested with dual client and breakpoints


# Changelog

Internal change only

code/_onclick/hud/human.dm

@@ -116,6 +116,10 @@
 		return
 	var/mob/living/carbon/human/H = mymob
 	var/mob/screenmob = viewer || H
+
+	if(!screenmob?.client)
+		return
+
 	if(!gear.len)
 		inventory_shown = FALSE
 		return //species without inv slots don't show items.
@@ -181,6 +185,9 @@
 	var/mob/living/carbon/human/H = mymob
 	var/mob/screenmob = viewer || H
 
+	if(!screenmob?.client)
+		return
+
 	if(H.hud_used)
 		if(H.hud_used.hud_shown)
 			if(H.s_store)