Fix String>>at:put: causes GPFault if argument has no instance variables or slots (#92)
Fix String>>at:put: causes GPFault if argument has no instance variables or slots (#92)

Dolphin issue #234
shoshanatech/Dolphin,jgfoster/Dolphin,objectarts/Dolphin,jgfoster/Dolphin,blairmcg/Dolphin,blairmcg/Dolphin,jgfoster/Dolphin,dolphinsmalltalk/DolphinVM,dolphinsmalltalk/Dolphin,objectarts/DolphinVM,objectarts/Dolphin,blairmcg/Dolphin,blairmcg/Dolphin,shoshanatech/Dolphin,shoshanatech/Dolphin,dolphinsmalltalk/DolphinVM,dolphinsmalltalk/Dolphin,objectarts/DolphinVM,dolphinsmalltalk/Dolphin,jgfoster/Dolphin,dolphinsmalltalk/Dolphin,shoshanatech/Dolphin