Fix runtime error with handle_heartbeat()
Fix runtime error with handle_heartbeat()

Fixes this runtime error:
//-------------------------------------------------\\
|| runtime error: Cannot read null.status          ||
|| proc name: handle heartbeat                     ||
|| (/mob/living/carbon/human/proc/handle_heartbeat)||
|| source file: life.dm,1648                       ||
\\-------------------------------------------------//

This was caused by no sanity check to make sure that
internal_organs_by_name actually returned an organ, therefore it was
attempting to read a null variable, which causes a runtime error.

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