The incident was reported by the Reddit user Turkeylord_2 who posted the images on Imgur and shared them on Reddit. Actually, the phone belonged to his roommate, who told the complete backstory later.
Notably, the user of the phone was not using the charger it came with. Instead, he was using a Samsung charger that came with his Samsung GS3 and the phone was lying next to him on the bed.
“I woke up to the sound of it sizzling and nasty fumes erupting from it,” he wrote.
Since GS3’s charger output is 1 A i.e. much lower than needed for Galaxy S6+, it doesn’t look like charger’s fault.
Meanwhile, owner of the phone has contacted Samsung and the company has promised to replace the phone and damaged property.